D. All rational numbers are integers
This rule applies to your expression
If all denominators of an expression are equal, simplify by adding the numerators
Thus; (numerators) 4 + 2 + 1 = 7
Simplifying your expression to a fraction of 7/8
Equivalent
8(51) = 8(50 + 1) = 8(50) + 8(1) = 400 + 8 = 408