Correct Answer: A
Explanation:
The word that goes into the blank describes superheroes, who the clue phrase describes as having "every detail of their musculatures would be visible through their clothing." Clearly, something like muscular is called for, and both thewy and sinewy fit the bill. The other four words don't fit: Superfluous means unnecessary, pneumatic means full of air, flocculent means covered in wool, and atrophied means shriveled due to disuse.