Answer:
D. The deer was not able to pick up the scent of the author.
Explanation:
The whole sentence reads
"The wind must have been right, because he kept on browsing the mossy ground." The deer kept on browsing the mossy ground because it did not pick up the scent of the man. Otherwise, it would have run away.
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')".
The man said, “I am going to the store.”
