Ron remembers Phil's birthday, which is in the same month as his, but fails to recall Alex's birthday, which is in a different month. Though both of them are his friends, Ron's ability to recall Phil's birthday but not Alex's can be best explained through the concept of:

The answer is self-reference effect.

Explanation:

The self-reference effect is described as the facility to remember something if we can relate it to ourselves. In the example, Ron can remember Phil's birthday more easily because it's in the same month as his. Another example is remembering names: you would be more likely to remember someone's name if it's similar, or the same, as yours.