Answer
This is a function.
Work Step by Step
A function is a correspondence between a first set, called the domain,
and a second set, called the range,
such that EACH member of the domain corresponds to EXACTLY ONE member of the range.
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We ask:
1. Does EACH player have to wear a number?
Yes.
2. Can a player wear more than one number?
No, he wears EXACTLY ONE.
This is a function.