The word Assiduous and the word Diligent 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 Assiduous with Diligent keeps the core idea intact without reversing the tone or changing the action, quality or state expressed by the target word. The alternatives Guarantee, Unavoidable and Stuff do not express the same central meaning as Assiduous. This distinction matters because competitive examinations often place one accurate synonym beside three familiar but contextually unsuitable words. If the sentence retains its essential meaning and grammatical structure, the synonym relationship is supported. Learners should revise Assiduous together with Diligent, a brief usage cue and the contrasting distractors.