Which function is a quadratic function?

u(x) is the only quadratic function.

Step-by-step explanation:

We are given four algebraic functions in x and we have to find which one is a quadratic function.

Let us arrange those polynomials in descending powers of x.

i)

The highest degree is 2.  Hence this is quadratic function.

ii)

The highest degree is 3 and hence this is cubic

iii)

The highest degree is 5 and hence this cannot be quadratic

iv)

The highest degree is 3 and hence this is cubic

Hence u(x) is the only quadratic function.