The inner function g(x) has x-13 in the denominator. Set it equal to 0 and solve for x
x-13 = 0
x-13+13 = 0+13
x = 13
If x = 13 then the denominator of g(x) will be zero. This is a problem since you cannot divide by zero
So x = 13 is not allowed as an input in the g(x) function. The same applies for the composite function as well.
So that's why the domain is the set of all real numbers x but x cannot equal 13
This translates to what choice D is saying, so that is the answer
