The way I solved the problem hasn't been mentioned here by anyone, which is slightly bugging me out.

The way I solved it was looking at the whole puzzle as a single picture. The two bottom rows (except for the middle column) have pluses. Thus the solution must have a plus. The two right columns (except for the middle row - a transposed pattern from the previous pattern) have squares; the solution must have a square. There's only two answers with both a square and a plus; I picked the one that seemed most intuitively correct.