The answer is (D). A number that has $a$ as the tens digit and $b$ as the units digit can be expressed as $10a + b$; the exercise asks to count the two-digit numbers for which $10a + b - (a + b) = 45$, that is, such that $9a = 45$. Such numbers are all those that have 5 as the tens digit: the integers from 50 to 59, which are 10.