Question

For what value of c is the relation a function (2,8),(12,3),(c,4),(-1,8),(0,3)

Answer 1

The value of c is { c | c ∈ R, c ∉ {-1,0,2,12} }.
c is a number such that it is not in the set {-1,0,2,12} else we end up with a domain value having multiple range values, contradicting with the definition of a function.