A function is a relation in which every input value x has at most one output value y.
Consider all options:
a. Relation {(a, b), (a, c), (a, d), (a, e)} is not a function, because input value a has four different output values b, c, d and e.
b. Relation {(a, b), (b, b), (c, b), (d, b)} is a function, because every input value a, b, c and d have at most one output value b.
c. Relation {(a, b), (c, d), (e, f), (g, h)} is a function, because every input value a, c, e and g have at most one output value b, d, f and g, respectively.
d. Relation {(a, a), (b, c), (c, c), (d, e)} is a function, because every input value a, b, c and d have at most one output value a, c, c and e, respectively.
Answer: correct choice is D
