Here is the original item:
With the huge number of confusing rules colliding in her
head,
(A) Cindy
took the quiz on fragments
(B) and
hoped,
(C) that
she would remember enough of the material to pass.
You wanted to do this:
With the huge number of confusing rules colliding in her
head. Cindy took the quiz on fragments and
hoped, that she would remember enough of the
material to pass.
The comma after head is correct. Substituting a period for that comma would make the opening phrase a fragment.
You might want to review the rules.