Final answer:
The domain of the function f(x) = 3x - 15 is all real numbers, which is represented by (-∞, +∞).
Step-by-step explanation:
The question asks to find the domain of the function f(x) = 3x - 15. The domain of a function is the set of all possible input values (x-values) for which the function is defined. Since there are no restrictions such as a denominator that could be zero or a square root that must be non-negative, the domain for this linear function is all real numbers. Therefore, the domain of f(x) = 3x - 15 is (-∞, +∞), which corresponds to option a.