The phrase "While answering questions" is an example of an introductory phrase, explaining information necessary to the context of the sentence, but outside of the main sentence, grammatically. These types of phrases must be followed by a comma in order to separate them from the rest of the sentence. The correct answer choice is, "While answering questions, Mathew was texting."

This sentence misuses a comma. While the conjunction "and" is present, there is no need for a comma because we do not yet have a list (three or more) of items. The comma should be omitted. Any other punctuation elsewhere is inappropriate since it would disturb the meaning of the sentence and does not belong, grammatically. The correct choice is, "collecting headphones and instruments."

The phrase "which has a bad reputation for being dangerous" is an example of a relative clause, which gives more information about its antecedent: nuclear energy. Any such phrase that occurs in the middle of a sentence must be introduced and completed with a comma. The correct answer choice is, "energy, which has a bad reputation for being dangerous, is actually."

The phrase "to clean his car, to buy groceries and to pay his bills" is a list of actions. All items in any sort of list/parallel words or phrases should be separated by commas. Additionally, a comma must come before the "and" because the actions "buy" and "pay" are not related in this case, so they should not be grouped together. The correct answer is, "to clean his car, to buy groceries, and to pay his bills." 

This sentence is missing a comma where needed. The phrase "Though acting may seem easy to most people" is an introductory phrase, explaining information necessary to the context of the sentence, but outside of the main sentence, grammatically. These types of phrases must be followed by a comma in order to separate them from the rest of the sentence. The clause "it takes much effort and practice" is fine and functions as an independent clause. No additional changes are necessary, so the correct choice is, "acting may seem easy to most people, it takes much effort and practice."

This sentence misuses a comma. The sentence is a simple sentence, so no punctuation of any sort is needed. The placement of the comma is strange because it is not joining clauses, being used in a list, or otherwise. In fact, the comma is in the middle of the same clause. The same issue lies with the usage of the other punctuation: there is only one clause in this sentence and any punctuation awkwardly splits it up. The correct choice is, "to learn about economics in order to gain a better understanding."

This sentence does not contain commas where necessary. The title, George Orwell's 1984 (1949), is an example of a parenthetical expression: a phrase that explains more about a particular word or phrase in the sentence, but is not introduced by a relative pronoun. These phrases must be introduced and completed with a comma in order to separate them from the main clause of the sentence. The correct choice is, "Kara's favorite novel, 1984, is a literary classic, according to most people."

This sentence misuses a comma. This sentence is a simple sentence, so no punctuation of any sort is needed. The comma should be omitted because it comes in the middle of an independent clause. The correct choice is, "Many elements on the periodic table."

This sentence is missing a comma. The sentence joins two independent clauses with the coordinate conjunction "but," so a comma should appear before "but." No additional punctuation is needed. The correct choice is, "competitions with one another, but many around them did not see the point."

This sentence is missing a comma where necessary. The phrase "Though Reese was an accountant by trade" is an example of an introductory phrase, which explains information necessary to the context of the sentence, but outside of the main sentence, grammatically. These types of phrases must be followed by a comma in order to separate them from the rest of the sentence. No additional punctuation is necessary. The correct choice is, "Reese was an accountant by trade, his passion was singing."