The correct option is C y=3x3+2x
A nonlinear function has the degree as 2 or greater than 2, but not less than 2.
Let's check the degree of all the functions.
(A) y=3x+2
The power of x in 3x and 2 are 1 and 0 respectively. So its highest power is 1.
→Linear function
(B) y=3x2+2x
The power of x in 3x2 and 2x are 2 and 1 respectively. So its highest power is 2.
→Nonlinear function
(C) y=3x3+2x
The power of x in 3x3 and 2x are 3 and 1 respectively. So its highest power is 3.
→Nonlinear function
(D) y=3x+2x
⇒y=5x
The power of x in 5x is 1.
→Linear function