Answer:
C.

Step-by-step explanation:
We have to find an equation which represents a nonlinear function.
Linear function: That function in which highest power of variable is 1.
Or
Polynomial of degree one is called linear function.
If degree of polynomial is greater than one then the function is called non-linear function.
Example : f(x)=x
It is linear because highest power of x is 1.
A.

It is linear function because degree of function is one.
B.

It is linear function because degree of function is one.
C.

It is non linear function because degree is 2 which is greater than 1.
D.

It is linear function because degree of given function is one.
Therefore, option C is true.