A linear function has to have a constant rate or change (called the slope).
Also, for a graph to represent a function, it has to have one and only one value of y for the same value of x of the domain.
In the case of graph A, this is a periodic function with a variable rate of change so it is a non linear function.
In the case of graph B (the circle), we have more than one value of y for the same value of x, so this graph does not correspond to a function.
In the case of graph C, we have an absolute value function, which have two different slopes depending on which part we are looking. Therefore it is a function but is non linear (although its parts are linear by themselves).
Answer:
Graph A --> Non linear
Graph B --> Not a function
Graph C --> Non linear
