This is not 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. --- We ask: 1. Does EACH member of the band play an instrument? Not necessarily. A singer, for example, needs not play an instrument. 2. Can a member of the band know how to play more than one instrument? Yes, so the EXACTLY ONE clause is also violated. This is not a function.