The correct option is D stout
Slender means (of a person or part of the body) gracefully thin. The opposite of slender is stout, or (of a person) rather fat or of heavy build. Hence, Option D is correct. Among the rest of the options, brave means courageous or bold, frail means weak or easily breakable, healthy means in a good physical or mental condition.These do not express this antonym, hence incorrect.