Which digit is neither prime nor composite?

Question

Four friends were eating lunch together at an outdoor restaurant.

Answer 1

You seem to have lost a big chunk of your question, but I suspect I know your answer all the same. The only single digit that is neither prime nor composite is 1 because a prime number has exactly two factors: itself and 1. Composites have more than 2. 1 is neither; it has exactly one factor.