The correct option is B sound
The word 'flimsy' refers to being insubstantial and easily damaged.
Option A, vigorous refers to being strong healthy and full of energy.
Option B, weak refers to something liable to break or easily damaged.
Option C, sound refers to something in good condition and not damaged.
Option D, salutary refers to something beneficial and producing good effects.
Thus, according to the meaning, the word 'sound' is the opposite of the word 'flimsy'.