Let's start with A (the incorrect answer)
4 minus 18 times x would be:
So 4 - 18x
B reads correctly because the product of 4 and x means we multiply the two together (getting 4x) - and then it says to decrease (or subtract) by 18.
So 4x - 18.
C reads correctly because 4 "times" is the same as multiply by x (which is 4x) and then it says minus 18.
So 4x - 18.