Answer choices "researchers have discovered several additional drugs that, when administered by a health care professional, has helped to reduce the risk of influenza" and "researchers have discovered several additional drugs that, when administered by a health care professional, helps in reducing the risk of influenza" contain subject-verb agreement errors because they use singular verbs—"helps" and "has helped"—rather than the plural verbs required by the plural noun "drugs."

The answer choices containing "they" as opposed to "researchers" are incorrect because they contain an ambiguous pronoun. We don't know to whom "they" refers.

The answer choice "researchers have discovered several additional drugs that, when administered by a health care professional, help to reduce the risk of influenza" is correct because it is the only answer choice that correctly avoids using an ambiguous pronoun and doesn't introduce any subject-verb agreement errors.

The correct choice is, "The company bought a new machine which had a reputation for incredible efficiency." All of the other sentences are ambiguous and confusing because the pronoun "its" could refer to either the company or the machine as it is used here. The use of "which" clears the confusion, since English grammar dictates that the antecedent of "which" and other relative pronouns come directly before the pronoun.

The best conveys the idea of this sentence is: "Though the parents found lecturing unpleasant, they lectured the children." The other choices are vague, since the pronouns used make it unclear as to who finds the lecturing unpleasant - the children, the parents, or both? It is also possible that no one specifically considers the lecturing unpleasant, but the sentence is making a statement that lecturing in general is unpleasant. The choice "Though the parents found lecturing unpleasant, they lectured the children" is very specific as to who finds the lecturing unpleasant and why.

This sentence contains an ambiguous pronoun. A reader would have no idea to whom "them" refers in this sentence. It is always important to clarify who or what the subject/object is before you are able to refer to it using a pronoun. The correct choice is, "with her friends."

The word "it" in this sentence is ambiguous. In this situation, it is possible for the "it" to refer either to the car or the part installed. Take, for example, that he installed a new radio or speakers in his car. It is entirely possible that the car would be able to work when these components are not working and vice versa. Perhaps while installing the radio/speakers, this person accidentally cut a wire that was vital to some other electrical components of the car. It is possible that the car may not start fully (i.e. to be able to drive), but perhaps the radio might still turn on - at least for a bit. So, it is important to clarify this; the best choice is, "the part did not work properly" since it is the only choice that is not vague.

This sentence contains an ambiguous pronoun. The word "it" could easily refer to the dog or the park in English. The choice that fixes this issue is, "his dog to the park, which was very big" because English grammar dictates that a relative pronoun (which) come directly after the word it modifies.

"Who" is not appropriate because this pronoun is only reserved for humans in English, and "that" is inappropriate because the semicolon before it dictates that both clauses must be independent (any phrase that follows "that" is dependent).

This sentence uses the pronoun "he," which is very ambiguous in this sentence. Does "he" refer to William or Toby? Changing the "he" to "who" fixes this issue because English grammar dictates that a relative pronoun appear after the word to which it is referring. "That" makes sense because William and Toby are humans (that is reserved for objects) and the other choices do not make any substantial changes to the original sentence. The answer is, "who was a very sociable individual."

This sentence contains many errors: foremost, it is very ambiguous in the usage of the pronoun in its current form. It is unclear whether the sentence means that the charts are loved or Trina's song. Additionally, passive voice is used in this sentence when active voice is better for a relatively complex sentence like this one (i.e. The subject must do the action of the sentence) in standard English. The correct choice is, "Trina's new song, which many people love, topped the charts."

The sentence contains an ambiguous pronoun. Technically, the word "which" could apply to both "the tree" and "the park" in this sentence. The word order needs to be reorganized in such a way that it is clear which words specifically refer to which nouns. The correct choice is, "The beautiful park's largest tree is as old as the city" as all of the other sentences contain some degree of ambiguity. This choice is the only one that is specific and clear.

Eliminate the choices containing ambiguous pronouns, as it is not clear to whom "their," "one," and "they" refers. Note that the correct answer adds the necessary auxiliary verb "do". The use of the noun in "how children's..." resolves any issues with ambiguity.