The development of new inventions in the field of space travel seemed rapid.

How does the word rapid function in the sentence?

predicate nominative

objective complement

predicate adjective

indirect object

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.