Answer
$y$ is the input which is the year;
$S(y)$ is the output which is the song in the top of the pop charts;
the relation is a function
Work Step by Step
In the given, the input variable is the year, $y$, in years, and the output is the song, $S(y)$, that was in the top of the pop charts in year $y$.
Since each year corresponds to a particular song in the top of the pop charts, then the given situation describes a function.
