To solve this question, we just need to insert 12 into the m position of each question and see if the equation holds true.
a. 4m = 40
4(12) = 40
48 = 40
48 obviously does not equal 40, so it is not choice A.
b. m + 20 = 42
12 + 20 = 42
32 = 42
Again, 32 isn't the same as 42, so not choice B either.
c. 4m = 48
4(12) = 48
48 = 48
It looks like this one is true, but let's solve D also just to make sure.
d. m - 4 = 9
12 - 4 = 9
8 = 9
This is false, since 8 does not equal 9.
Therefore, choice C (4m = 48) is the correct answer.
Hope that helped! =)