A function is a rule that tells you how to associate inputs and outputs.
In other words, expressing y as a function of x means: "tell me the value for x, and I will use a formula to compute the value for y".
This implies that, when a certain input x is chosen, there can only be a unique y value as output.
So, the only feasible option is C. All other options assign two different outputs to the same input. For example, in option A, you have the couples (-1, 3) and (-1, 6), which can't happen in a function: you would be claiming that, when you choose -1 as input, the output would be 3, but also 6, which is impossible. The output must be unique.
The same goes with option B (you assign two outputs to the same input 0) and option D (the input 2 has two outputs)
