Answer
a) All are one-to-one lest someone shares a phone number.
b) All are one-to-one (no id is the same).
c) If everyone has a different grade.
d) If everyone is from a different home town.
Work Step by Step
A function is one to one so long as there are a matching number of elements in each set that do not share the same domain (no vertical line). If each element is potentially unique it is one-to-one. If there is a chance that the elements will be duplicated it is not one-to-one.
