In terms of a fraction equaling out to 1, the numerator and denominator have to be the exact same number. With that in mind:
B is incorrect because 17 is one number less than 18.
C is incorrect because 1 is four numbers less than what would equal a whole.
D is incorrect because the numerator is greater than the denominator, which would mean that the fraction 7/4 is greater than a whole.
A, however, is correct; 31 = 31, meaning they are the same number. If the numerator and denominator are equal, we know that we have a whole. Therefore, A is correct.