Here is the item again:
Citing writer's block as the reason
(A) that she didn't have her
essay, (B)
Yao received no sympathy from Mrs. Santos.
(C) Who will only accept
"breather's block" as a legitimate excuse for late work.
You correctly realized that the period after Mrs. Santos creates a fragment. Who will only accept "breather's block" ... is a subordinate clause, so it 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 didn't have her essay, Yao
received no sympathy from Mrs. 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 Mrs. 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!