Which of the following is an odd function?

An odd function satisfies f(-x) = -f(x). For f(x) = -x, replacing x with -x gives f(-x) = x, which equals -f(x), so this function is odd. Choice B includes an even-power term, and choice D also includes an even-power term, so neither is odd; the square-root function in C is not defined for all negative x.

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