Evidence, including bedding and other domestic artifacts, shows that t

This is a good question--there's a lot of which/where/when trickery in Sentence Correction.

The basic principle at work here is that the preposition + which modifier form is extremely versatile. Consider these two examples:

I have never been back to the city where I was born.
I have never been back to the city in which I was born.

The first sentence might seem like the natural choice, as where can definitely be used to refer to the place noun city. However, the second sentence is also correct, because it's fair to say that I was born in the city. To put it another way, because the which in the second sentence can sensibly stand in for the city, which also happens to be the nearest preceding noun, it's perfectly acceptable.

So too in this question's answer choice A. Because the which is near camp and can sensibly refer to it, it's totally fine. And while one could definitely refer to camp with a where, the GMAT is really averse to preposition + where/when constructions. It strongly prefers preposition + which.

I hope that helps!

Ryan