You can look at one of the points and see what happened to their x and y values. I'm going to take a look at point H.
It went from (2, 6) to (1, 3). This is a dilation factor of 1/2 because notice that you have to multiply the values (2, 6) by 1/2 to get (1, 3). So, your answer is C, and not A.