Which is a quadratic function?

A quadratic function has a nonzero term with x squared and no higher power of x. Choice C, f(x) = 5x² − 4x + 5, has a nonzero x² term, so it is quadratic. In choice B, the coefficient of x² is 0, so that term disappears; choice D has an x³ term and is cubic.

AI-written and checked against the verified answer.