We can't see which is underlined but I'm assuming it's hid based on the question. It's B) Past. The reason is that hid refers to something he has already done. So it's in the past. And past perfect means that the wording is a bit more complex. It would only be past perfect if it said 'The mouse had hidden under there before." so they have two words saying it's hid there, had and hidden.
Hope this helps!