Take a look at the sentence again:
__________, a dirty hamster cage, and a glass of old milk all contribute
to the unbearable stench emanating from Wilson's bedroom.
The blank requires a noun phrase. You chose sweaty gym socks that smell, which is a noun phrase followed by an unnecessary adjective clause. You thus have a parallelism error. Before you continue, review the rules for parallel structure.
Go back to the sentence to try again.