Answer:
The answer is 9, but it is not included on the possible answer list.
Explanation:
Lets write the expression as:
18/4 = x*(1/2) [x, the unknown, is used instead of ?]
4.5 = x*(1/2)
2*4.5 = x
x = 9
This is telling us that 4.5 is equal to 9 times (1/2).
9 does not appear on the list, so the problem probably has a typo somewhere.