First of all what is the size of each grid? Each one has 10 rows and 10 columns so that's 10*10=100 little squares. For every 25 squares there are 4 purple marbles. But we have 100 squares which is 4*25. So we must mark 4*4=16 squares to represent 4/25. The answer must be A. 16/100=4/25=16 per cent which means 16 in a hundred. I suppose any 16 squares could have been marked but it makes sense to put them all together.