When separating items in series, a comma must be placed before the conjunction be placed after the conjunction be placed both before and after the conjunction not be placed before the conjunction

This is what a series looks like:
one, two, three, four, and five
The conjunction there is the word "and."
No comma should come after it, but a comma may come before it.

When I say may, I mean it's completely optional. You can also write
one, two, three, four and five
and still be correct.
That optional comma is called an Oxford comma.

Whether or not the answer is "before the conjunction" or "not before the conjunction" is entirely up to your teacher, though most would probably want you to use one. In this case, I would say "before the conjunction".

When separating items in a series, a comma must be placed before the conjunction.