Explanation
In this vocabulary item, Guarantee is the closest synonym of Ensure, so option A is correct. The word Ensure and the word Guarantee 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 Ensure with Guarantee keeps the core idea intact without reversing the tone or changing the action, quality or state expressed by the target word. The alternatives Intimidate, Cure and Climb do not express the same central meaning as Ensure. This distinction matters because competitive examinations often place one accurate synonym beside three familiar but contextually unsuitable words. To verify Ensure independently, identify its part of speech, state a short definition, and place both Ensure and Guarantee in the same simple sentence. If the sentence retains its essential meaning and grammatical structure, the synonym relationship is supported. Learners should revise Ensure together with Guarantee, a brief usage cue and the contrasting distractors.