S is not a function.
A function is a set of ordered pairs where each input (x-value) corresponds to one and only one output (y-value). In this set, S, there are multiple inputs (a, c, d) that correspond to the same output (3). This means that the input 'a' is not uniquely related to output '5' and 'b' is not uniquely related to output '3', etc. Therefore, S does not qualify as a function.