Take a look at the passage again:
Raja knew that his roommate Tina had cupcakes hidden
in the apartment. He looked everywhere. Including in the clothes
hamper, on top of the bookcases, behind the sofa—even under his own bed!—all
to no avail.
You chose the first part, Raja knew that his roommate Tina had cupcakes hidden in the apartment. This part has a main clause: Raja is the subject; knew is the verb. A fragment will not have a main clause.
Go back to the passage to try again.