The way I calculated this was by doing 25/1000 * 6.02 * 10^23 and then recognising that there were three ions, 2 Na + and and So4 2- ions, so the final operation was:
25/1000 * 6.02 * 10^23 * 3 = 2.26 x 10^21 (A)
But this is apparently wrong and the correct answer is (B) as I should have not multiplied by three at all and left the sum as