Which function is a quadratic function?

f (x) = 2x2

g (x) = 5x3

k (x) = - x

b (x) = - 9x4

F (x) is the only quadratic function here. Any term with a degree of 2 starting the equation is a quadratic.

Quadratic = 2nd degrree, means highest exponent of placeholder is 2

f (x) = 2x^2 is 2 exposnnet, that is quadratic

g (x) = 5x^3, 3rd degree, cubic

k (x) = - x, 1st degree, linear

b (x) = - 9x^4, 4th degree, quartic

answer if f (x)