Here is the item again:
Citing writer's block as the reason
(A) that
she did not have her essay,
(B) Yao received
no sympathy from Professor Santos.
(C) Who will
only accept "breather's block" as a legitimate excuse for
late work.
You correctly realized that the period after Professor Santos creates a fragment. Who will only accept "breather's block" ... is a subordinate clause, which cannot stand alone as a complete sentence.
To fix this fragment, you chose to do this:
Citing writer's block as the reason that she did not have her essay,
Yao received no sympathy from Professor Santos,
who will only accept "breather's block" as a legitimate
excuse for late work.
To fix a subordinate clause fragment, you usually use Punctuation Rule 1:
Main Clause + Ø + Subordinate Clause.
Who, a relative pronoun, is a special case. Because Professor Santos is a specific noun, you will need a comma to connect the two parts. Check out Comma Tip 5 for additional information.
Rock on!