Choose the alternative which can replace the bold word without changing the meaning of the sentence.
The scientist was known for his perspicacious observations, which led to groundbreaking discoveries.

In the sentence, "The scientist was known for his perspicacious observations, which led to groundbreaking discoveries," the word "perspicacious" means having keen insight or sharp understanding.
Here are the options provided:
Dull – This is the opposite of perspicacious, meaning lacking sharpness or insight. Incorrect.
Keen – This is synonymous with perspicacious, meaning highly perceptive or having sharp insight. Correct.
Unclear – This means vague or not easily understood, which is the opposite of perspicacious. Incorrect.
Hesitant – This means uncertain or indecisive, which does not relate to keen observation. Incorrect.