The correct answer is a.: specific.
This structure (this +adj) requires an adjective and the only adjective is in a., the other options are verbs or nouns and are not correct.
C. The platypus, which is a mammal, has a bill like a duck and feet like an otter.
"which is a mammal" is an adjective clause. An adjective clause is used to describe a noun and usually begins with one of these words: that, when, where, who, whom, whose, which, and why. In this sentence, the adjective clause begins with "which" and describes the platypus, which is a noun.
.org does because org is short for organisation.
