Take a look at the sentence again:
Horatio tickled the pads of Samson's feet, blew in his ears, and __________,
but the ancient German shepherd refused to leave the sofa for a walk in
the rain.
The blank needs to begin with a simple past tense verb. You chose there was the promise of a liver-flavored cookie, which is a main clause and thus causes a parallelism error. Before you continue, review the rules for parallel structure.
Go back to the sentence to try again.