Management comes up with these contradictory requirements all the time :)
But, really, this just looks like they mean that there must be SOMETHING after the X, but it could be a, b, OR c, or more than one of those, in relative order.
So, Xa is ok, so is Xb, Xc, Xab, Xac, Xbc, and Xabc. Just X by itself is invalid - it's not allowed to be empty.
I seem to recall such types of ambiguity to be common both among teachers and managers. Something about not having to implement it themselves combined with "I know what I mean. Why don't you?".
And, based on my tests,
should do the job. But if the OP doesn't understand why while trying to hand it in as their own work, they'll find that they're going to get further and further behind as their course progresses.