Final answer:
The domain of f(x) = 3x is all real numbers.
Step-by-step explanation:
The domain of a function is the set of all possible input values or x-values. For the function f(x) = 3x, the domain is all real numbers. This means that the function can accept any real number as an input.
The graph of f(x) = 3x is a straight line that passes through the origin (0,0) and has a slope of 3. Since there are no restrictions on the x-values, the domain is all real numbers.
Therefore, option a. all real numbers is the correct answer.