When comparing more than two items, you should use superlative adjectives. So, in this case, you should use the word "best."

This sentence contains a superlative adjective error. When forming a superlative in English, the pattern "most + positive form" or "positive form + -est" is used (e.g. "most great" or "greatest"). The word "most" is not mixed with the superlative "____________-est." It is also clear from the context of the sentence that the company thinks that the employees are the best option, not just better than some alternative, so the superlative form is needed, not the comparative forms "greater" or "more great." The correct answer is, "the company believed that its employees were its greatest asset."

This sentence misuses the adjective "great." The form "more greater" does not exist in English. Either "more great" or "greater" would be correct, but the two cannot be mixed. Furthermore, you cannot use the superlative form (e.g. greatest/best) with the word "than." The correct choice is, "Nathan's software is more great than competing software."

This sentence contains an error in the form of the comparative adjective. When forming the comparative form of an adjective, use either the "more + adjective (i.e. more cute)" form or "adjective + -er (i.e. cuter)" form. You cannot mix the two and cannot use any words that indicate a superlative (i.e. most). The correct choice is, "this actress was more cute than all of the others."

This sentence contains an error in the usage of the superlative. Specifically, superlatives are formed either by adding -est to the end of an adjective, or adding the word most before it; both options cannot be used together. The correct choice is, "tallest."

This sentence contains an error in the usage of the superlative. The word "than" indicates that a comparison is occurring in the sentence; therefore, a comparative adjective is necessary. The correct form is "larger." Note that the comparative adjective form in English is formed by either adding -er to the end or more to the beginning of the word. Using both is unacceptable. The correct choice is, "a book larger than his last one."

The use of "they're" is incorrect in the sentence. "They're" is a contraction for "they are," while the sentence needs the third person plural possessive pronoun, "their." "Fewer people threw their support" is the correct answer choice.

The central issue here is the incorrect superlative "most happy". The correct superlative is happiest. All of the syntactic modifications in the other answer choices either create agreement issues or simply fail to resolve the central issue.

This sentence uses the wrong form of "grateful." The correct comparative form of "grateful" is "more grateful. "Gratefulled" is not a word and "gratefully" is an adverb, meaning it describes adjectives and verbs, not nouns (i.e. Hank). The correct choice is "more grateful."

This sentence does not use the correct form of the word "great." It is clear from the context of the sentence that the superlative form is necessary, which could either be "greatest" or "most great." The former is the only available choice, however. The correct choice is, "considered his students the greatest." It is not necessary to add a noun after "greatest" as the adjective can be used as a substantive (to take the place of a noun, implying "greatest (students)").