The birthday attacks are called so because we can only have 365 birthdays (oh well, excluding 2/29). So, if there are 366 people in a place at least two of them have the same birthday - thus causing collision(s). Same things happens in hash algorithm. The people in birthday attack is the same thing as

The correct approach would be "the input in the hash algorithm".

Explanation:

The value that will also be determined because after value or importance is based seems to be hash. The birthday sometimes used measure the hash would have been the key. The algorithm would have a certain code as well as a pseudocode. Therefore the input upon whom hash would be evaluated is individuals.