The opposite to the word "placid" is "agitated". The correct option among all the options given in the question is option "A". Before searching for the antonym of any word, it is important to know the synonym of the given word. In case of the word given in the question which is “Placid”, the meaning or the synonym is calm and peaceful. Once this is known then finding the antonym becomes very easy. The antonym of “placid” comes out to be “agitated”.