Do you remember the reading for 開ける, which you learned in level 10? That's right, it's あける! So if you know the reading of that word (I hope you do!) then you can apply that knowledge here.
Just a quick side note — 開く can also be read as ひらく, but we're going to focus on the あく reading here. Since ひらく has slightly different meanings, we won't accept it as a reading for this word, so be sure to enter あく!
What time does this shop open?
Your fly is open.
"Excuse me, is the Tofugu Museum open tomorrow?" "Yes, it's open."