I'd say that D is the right answer.
Your answer should be WHO.
Think of it like this: You wouldn't say "The boy it (a variant of 'which')", because it simply just sounds odd. Instead, you're wired to think "The boy, he (an unknown name, so it would be 'who')".
A. Recognizing there is a problem.
(I think)
Okay, so I think your answer is <span>D. He reads the instructions below the door.
I'm pretty sure I'm right, but good luck my dude.</span>
