On the first attempt, the probability of pulling red is 2/4 or 1/2 since there are 2 reds out of the four socks available. Same goes for the next two attempts since the sock pulled is put back every time. To determine the probability (P) asked, multiply the three probabilities.
P = (1/2) x (1/2) x (1/2) = 1/8
The answer is 1/8.