The correct option is B stout
The word 'slender' refers to being gracefully thin.
Option A, strong refers to being able to withstand force, pressure or wear.
Option B, well-built refers to having a strong or sturdy physique.
Option C, stout refers to being rather fat or of heavy build.
Option D, slim refers to being gracefully thin or slender.
Thus, according to the meaning, the word 'stout' is the opposite of the word 'slender'.