Which sentence does not contain any errors?
a.
how much is the waitress tip?
b.
how much is the waitress' tip?
c.
how much is the waitresses tip?
d.
how much is the waitress's tip?

D, I think.

If you read A out loud, it obviously doesn't flow correctly.

B's usage of the apostrophe is incorrect; an apostrophe after the 's' is only appropriate for possessive plural nouns - or in this case, more than one waitress - but in the sentence there isn't any indication of multiple waitresses. In fact, it says "How much is the waitress' tip?", implying that there is only one, singular waitress.

C's usage of 'es' at the end of waitresses, again, implies that there are multiple waitresses, but there is no indication of this. The usage of "is" in the sentence (How much is the waitresses tip?) implies that there is only one waitress.

That leaves us with D, which doesn't contain any errors (as far as I know). Hope this helps! :)