The graphs are:
Graph 1: Non-linear.
Graph 2: Linear.
Graph 3: Non-linear.
How to know if a function is linear?
This is ratter simple. If a function is linear, then the graph is just a straight line, like we can see in the graph of the middle.
The other two are clearly not linear, the left one is a cubic function and the right one is a quadratic, so these are non linear-
The only correct option is the middle graph.