Which of the following is an odd function?

An odd function satisfies f(−x) = −f(x). In choice D, f(x) = 6x³ + 2x, so f(−x) = 6(−x)³ + 2(−x) = −6x³ − 2x = −f(x). Therefore, 6x³ + 2x is the odd function.

AI-written and checked against the verified answer.