The old man asserted that the whole art of medicine lay in judicious poisoning.

Answer: Option A

Wise : having or showing experience, knowledge, and good judgement.
Clever : quick to understand, learn, and devise or apply ideas; intelligent.
Cautious : (of a person) careful to avoid potential problems or dangers.
Careful : making sure of avoiding potential danger, mishap, or harm; cautious.
Judicious : having, showing, or done with good judgement or sense.

Synonym of Judicious is Wise