You're on it
vincent.m!
There's a term shift from "player who specializes in defense" to "player in a defense position". The first one, which matches with the stimulus, gives us a characteristic of the player. The second one gives us a characteristic of the position itself!
The incorrect answer would apply to any player
in a defense position, whether they were a defensive specialist, an offensive specialist, or your little sister! The correct answer applies to all defensive specialists
regardless of what position they are put in to play.
It's a very subtle language shift that actually has significant implications! It's questions like this that will help train you to get more and more sensitive to this variations in language.
Keep up the great work!