Take a look at the passage again:
Rocking and thrashing like a wild horse that cowboys had lassoed. The washer
complained about its overloaded tub. At the other
end of the Laundromat, Bobby quietly read an old magazine, pretending that
it was not his machine.
You chose the third part, At the other end of the Laundromat, Bobby quietly read an old magazine, pretending that it was not his machine. This part has a main clause: Bobby is the subject; read is the verb. A fragment will not have a main clause.
Go back to the passage to try again.