Which equation is not a linear function

Then where's the chöices??

You don't have a chöices so then my answer is

Algebraically, linear functions are polynomials with highest exponent equal to 1 or of the form y = c where c is constant. Nonlinear functions are all other functions. An example of a nonlinear function is y = x^2. This is nonlinear because, although it is a polynomial, its highest exponent is 2, not 1.

i'm not sure i quite understand, could you provide more information?