Answer:
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.