Here is the original item:
One day in class, Shantrell rested her head against the
wall (A)
and discovered
(B) that she could hear
the classical music that Professor Nguyen played in his office on the other
side. (C)
The music promptly put her to sleep.
You wanted to do this:
One day in class, Shantrell rested her head against the wall and discovered
that she could hear the classical music that Professor Nguyen played in
his office on the other side. Which promptly
put her to sleep.
Replacing The music with which makes a fragment. Which introduces a subordinate clause that cannot stand alone as a complete sentence.
You might want to review the rules.