Final answer:
The domain of the linear function f(x) = x + 13 consists of all real numbers, as there are no restrictions on the input value x.
Step-by-step explanation:
The domain of a function refers to the set of all possible input values that the function can accept. For the linear function f(x) = x + 13, there are no restrictions on the value that x can take. This means that x can be any real number, whether it's positive, negative, or zero. Consequently, the correct answer to this question is D. All real numbers.