The correct answer is predicate adjective.
A predicate adjective is the first adjective found after a linking verb (used to link, or connect two parts of a sentence). In the sentence above, the linking verb is seemed and the first (and the only) adjective after it is the word rapid.
There are no objects in the sentence so B and D are incorrect, and given that rapid is an adjective rather than a noun, A is also incorrect.
