Explanation
In this vocabulary item, Trivial is the closest synonym of Frivolous, so option D is correct. The word Frivolous and the word Trivial share the same central sense in the context tested by the question, although usage, tone and grammatical form should still be checked before substituting one for the other. A synonym is not required to be identical in every possible sentence; it must preserve the intended meaning in the relevant context. Here, replacing Frivolous with Trivial keeps the core idea intact without reversing the tone or changing the action, quality or state expressed by the target word. The alternatives Clear, Agree and Keen do not express the same central meaning as Frivolous. This distinction matters because competitive examinations often place one accurate synonym beside three familiar but contextually unsuitable words. To verify Frivolous independently, identify its part of speech, state a short definition, and place both Frivolous and Trivial in the same simple sentence. If the sentence retains its essential meaning and grammatical structure, the synonym relationship is supported. Learners should revise Frivolous together with Trivial, a brief usage cue and the contrasting distractors.