The correct option is
C Slim
The word 'rotund' refers to a person who is large and plump.
Option A, feeble refers to a lack of physical strength, especially as a result of age and illness.
Option B, healthy refers to a god physical and mental health condition.
Option C, slim indicates a thin and slender build.
Option D, weak refers to having little physical strength and energy.
Thus, according to the meaning, the word 'slim' is the most different in meaning from the word 'rotund'.