The correct answer is A. Unusual
Step-by-step explanation:
In grammar, an adjective is a word that describes or modifies a noun or pronoun, different from an adverb that modifies verbs or adverbs. Adjectives can be identified because they define nouns and are usually located befor them or after them if a linking verb is used to link the subject wih an adjective that describes it. In the case of sentence "Occurrences of major earthquakes are considered unusual, but minor tremors happen frequently" the words "major", "unusual" and "minor" are all adjectives because they describe or define earthquakes/tremors. Thus, from the options provided the only one that is an adjective is "unusual".