Answer:
foot-in-the-door
Explanation:
Foot-in-the-door (FITD) technique is a tactic in which a person is made to give his consent for a larger request by making him agree to a modest or small appeal in the initial stage. In the above case, Samuel makes Katie donate him a $15 by first asking her to wear a ribbon. In the initial part, Katie didn't find wearing a ribbon a much to do. Her agreement to this little job made her donate the money too.