Answer: A!
Step-by-step explanation: The range of f(x) = 3x + 9 is all real numbers. Notice that f(x) = 3x + 9 is a polynomial function with a highest exponent of 1 on the variable, and it is in the form f(x) = mx + b with m = 3 and b = 9. Therefore, f(x) satisfies the definition of a non-horizontal linear function.