Take a look at the item again:
Once classes were over, (A)
Maria used to meet her lab partner Chris at the diner near
campus (B)
until she discovered that Chris preferred,
(C) to flirt with the
waitresses instead of discussing their chemistry homework.
You wanted to do this:
Once classes were over Maria used to meet
her lab partner Chris at the diner near campus until she discovered that
Chris preferred, to flirt with the waitresses instead of discussing their
chemistry homework.
You need a comma after over. Once classes were over is a subordinate clause, and when you begin a sentence with one, you must use a comma to attach it.
You might want to review the rules.
Go back to the sentence to try again.