When you reverse the digits in a certain two-digit number you increase its value by 18. what is the number if the sum of its digits is 12?

Let's call the number ab with digits a and b

a+b=12 => b=12-a
a*10+b + 18 = b*10+a => 9a + 18 = 9b

9a + 18 = 9(12-a) => 18a = 108-18 = 90 => a=5

b = 12-5 = 7

The number is 57. Reversed is 75 and 57+18 is indeed 75.

The answer would be c