I think the answer is C. 5
If you get 6.4grams of CuSO4 out of 10 grams thats like saying that CuSO4 is 6.4/10 or 64% of the total mass. So (total mass)(percentage) = mass of CuSO4(which is the mass of Cu+S+4O or 159grams/mol). (x total mass) * (0.64) = 159grams of CuSO4 We find the total mass to be 249grams.
So now we know the total mass as 249grams. If there is 64% CuSO4 there must be 100-64=36% H2O. So 249 * 0.36 = 89grams. Find the answer which equals 89grams or so. 5 H2O is (18*5) = 90 grams.