Discussing this with some friends, it was suggested that the correct choice is
5, ie. bottom row in the middle
the puzzle is a form of simple addition. The first two pictures are added together, producing the third picture. Overlapping squares produce a large square, but beyond that there is no "overflow" so a small square plus a large square still produces a single large square. Note that this solution only works without question when reading the puzzle left-to-right; when reading top-to-bottom, the meaning of the alignment of the picture (whether the squares are on top of or below the horizontal line) is unclear.
I gamed the test a little by repeating it and seeing how different choices for this question affect the scoring, and I think this answer could be correct, but am not completely sure.
EDIT: I gamed the test a bit more by selecting the top left choice on every question until 29, and selecting the correct choice after that; this way the test gives a score of 91 if all choices are correct, and 90 if there is one mistake after the 29th question. I went through all the possibilities for question 34, and it appears that there are two correct answers:
3 (top right) and 5 (bottom middle)
Which means that both this answer and the one by prog_SAHIL (and Jan Ivan, who also suggested the same choice albeit with a more mathematically sound reasoning!) are correct.