The correct option is B Vigorous
The word 'sluggish' refers to lacking energy or alertness.
Option A, aggressive refers to ready or likely to attack or confront.
Option B, optimistic refers to being hopeful and confident about the future.
Option C, vigorous refers to being strong, healthy and full of energy.
Option D, budding refers to beginning and showing signs of promise in a particular sphere.
Thus, according to the meaning, the word 'vigorous' is the farthest in meaning from the word 'sluggish'.