Explanation:
I am severely tempted to say "not enough information given".
why ?
while the government makes a lot of effort to ensure that every living American has an unique social security number, they're have been known mistakes.
plus, social security numbers of supposedly dead people are being reused for "new" living people to some degree (depending on the number coding algorithm that includes areas, the 2-digit number of the registration year, the running number of the person being registered in that area in that year, ...). and then illegal stolen identities,...
in theory, I guess, your teacher was aiming for unique social security numbers, but in practice there is not 100% guarantee.
then : international employees (in other countries) might not have a social security number in the same sense or with different number structures or ...
I also had at least one case of a colleague of mine at a large international company, who actually got 2 different salaries from 2 different country organizations of that company. it was not a possible situation according to our company regulations, but when certain people do not pay attention to certain details, things like that can happen.
but, if we consider only the most simple case and in its intention, a purely USA company, with all legal employees (without any "statistical flaw"), then yes, it is a function, as every social security number (x-value) is unique and has exactly one salary (y-value) assigned.
in reality companies try to escape these special cases by assigning truly unique employee IDs to the employees, and every employee ID has exactly one associated salary.
that is then for sure a function.
remember, a relationship of 2 sets (x, y) of e.g. numbers is a function, only if for every member of x there is exactly one associated value of y.
multiple different x-values can have the same y-value. but no x-value can have more than one y-value (in our example, no social security number can have more than one salary).
this also means an x- value without an associated y- value is not valid either. if that happens, it is not a function either. in this case such x-values are removed from the set x to make the relationship a function again.