"Their" is a possessive pronoun that indicates that an object belongs to several owners. However, this sentence refers to a single actress, so the lines only belong to one person. Therefore, the best pronoun to use is "her" because it matches the singular nature of the subject: she should study her lines. The correct sentence would be "An actress who struggles with memorization should study her lines daily."

"Their" is a possessive pronoun that indicates that someone belongs to several people. This sentence, however, refers to a single subject (a professor), so the best pronoun to use is one that signifies that an object belongs to one person: in this case, his students. Even though "students" is plural, the plurality of the object does not impact the possessive pronoun. Only the plurality of the subject matters, so "his" is the correct pronoun, making the corrected sentence "A professor who assigns an unreasonable amount of reading for his students may never earn the class's approval."

In this sentence, the pronoun “it” (singular) refers to “witty comebacks” (plural). These words should agree in number. The corrected sentence reads, "Penelope always thought of the most fabulous witty comebacks two hours after the situation in which she could have used them."

In the above sentence, the clause "no merchant should ever be knighted" is being modified by the phrase "no matter how great a difference they made for the country," which means that the pronoun "they" has the antecedent "merchant." Since "merchant" is singular, the pronoun(s) used to replace "merchant" should also be singular—in this case, "he or she."

In this sentence, an error occurs when the possessive plural pronoun "their" is used to refer to a singular masculine antecedent, "Bob." Instead of using the plural possessive pronoun "their," one should use the singular masculine possessive pronoun "his."

The plural pronoun "they" refers back to the subject of the first part of the sentence, "The toucan." Since this noun is singular, the pronoun should be too. Changing "they" to "it" corrects the sentence's error. The corrected sentence reads, "The toucan is primarily recognized by its bright colors; it is painted a mixture of vibrant red, yellow, and blue."

"Their" should be changed to "its." The pronoun must agree with the noun that precedes it, and in this case the noun, "committee," is singular.

Note: Standardized tests in the United States tend to treat collective nouns (such as "committee," "team," or "jury") as singular. While it is grammatically correct to treat them as plural if all members of the collective are doing different things, they are treated as singular if all group members are doing the same thing (as in this case, all on the committee are deciding to interview), which is usually the case on standardized tests.

Since Professor Clarke is speaking to both Daisy and Margaret, the pronoun "she" is used incorrectly here—whom is "she" referring to? "She" should be changed to "they" to reflect that both women are being spoken to by Professor Clarke.

Because Paul and Tom are a unified unit, they become a group of more than one. As such, the plural pronoun "they" should be used to refer to them, meaning that "he" in the sentence is incorrect.

The correct version of this sentence would read: "Paul and Tom had a great time working together; they make a good team."

Although there is a "nor" separating Paula and Mark, they are still being treated as a single, plural unit in the second clause. Therefore, the sentence is correct as written. "They" and "their" correspond to the plural antecedent group of "Paula and Mark."