Superlatives can be used when comparing three items, but when comparing only two objects, like the two cookies here), you must use comparative adjectives. So, the superlative adjective "sweetest" should be changed to the comparative adjective "sweeter" to correct the sentence's error.

Because two novels are being compared, Mary can only argue that one is "better" than the other. However, the sentences uses the superlative adjective "best" instead of the comparative adjective "better." The sentence's error could be corrected by changing "best" to "better."

This sentence compares three different sports: soccer, basketball, and swimming. Whenever three or more things are compared, a superlative adjective is used in the comparison, not a comparative adjective, which is only used for comparisons between two things. So, "faster," a comparative adjective, needs to be changed to "fastest" to correct the sentence's error.

This sentence uses a comparison adjective (“more”) when comparing three things: "walking," "taking public transportation," and "driving." A superlative (“most”) would be grammatically correct. The corrected sentence reads: "Walking and taking public transportation are also options, but driving remains the most popular way to arrive at the festival."

This sentence uses the comparative “more,” which should be used when comparing only two things. As this sentence compares all professions, the superlative “most” should be used. The corrected sentence reads, "He could have chosen any profession in the world, but he decided that accounting was the most interesting."

When two items are being compared, “more” is the appropriate word to use, while “most” is appropriate for comparisons involving three or more items. Here, two items are being compared: "black tea and coffee." Therefore, “more” is the appropriate word to use, not the phrase “the most.”

Remember to use the comparative form of an adjective (the form that ends in "-er" or is preceded by "more") when comparing only two items, and to use the superlative form of an adjective (the form that ends in "-est" or is preceded by "most") when comparing more than two items. This sentence tells us that there are three items being compared, so "heaviest" and "most pliable" are correct here. Only "stronger" is in the comparative form when it should be in the superlative form, so it contains the sentence's error. The answer choice "necessary to" is a correct use of the idiom.

The phrase "more busier" is redundant—it should just be "busier," which implies the "more" aspect already. As seems like an attractive choice, but the phrase "in demand as magazine editor" displays proper idiomatic usage.

We use the word "less" to describe a quantity that doesn't have discrete parts. For example, we can say that we have "less money," because money doesn't have an inherent discrete unit; however, we cannot say that we have "less dollars" because a dollar is a discrete unit of dollars. In the case of the sentence above, since ideas have a discrete unit of a single idea, we say "fewer ideas," not "less ideas."

When two subjects are being compared (such as the twins in this sentence), the appropriate preposition to use for comparison is "between." "Among" is only appropriate when three or more subjects are being compared.