Which of the following functions is not linear?

A linear function can have terms which are made of numbers or numbers multiplied by a single variable with exponent 1 (usually not written). If you have variables multiplied together, exponents other than 1 on variables, variables in the denominator, or variables in a square root, it is not linear.
Look at the choices.
Option A. has x^2. That is not linear.
All others are linear.

Idon’t see the picture