Its absolutely 'D' as per the above commenters said but the puzzle sits wrong with me as in this kind of grid, there should also be some relationship on the verticals and diagonals too. As the horizontal lines from which we all got the answer to have a relationship but yet, there is nothing signified, then the other axis' should too in my opinion!
Best wishes mate and thanks for the daily fun :-)
RE: Maths Brain Teasers 38:: Can You Decode iT???