The quick answer is C does not represent a function. It has two y values for x = 2. These two points are (2,1) and (2,3). That eliminates C. No x can have 2 y values and be called a function. A function can have many x vales for a single y and still be a function.
A has all different x values, so it could be classed as a function. There are no values where x has more than 1 y value.
B also has all different x values. It too is a function.
