The answer is divide by 2.
128 ÷ 2 = 64
64 ÷ 2 = 32
32 ÷ 2 = 16
16 ÷ 2 = 8
The others can't work because:
128 ÷ 4 = 32 (This is not the next term, this is the one after - therefore it's wrong)
128 - 8 = 120 (This isn't even a term in the pattern.)
128 - 64 = 64
64 - 64 = 0 (This only works for the first term, after that, you get 0 - not a term in the pattern.)
