Answer:
Rule of Reciprocrity
Explanation:
Rule of Reciprocrity is a term in social psychology that describes a situation in which an individual returns a favour or pay back what was done to him in the past.
Hence, in this case, in the movie White Christmas, Phil Davis (Danny Kaye) used RULE OF RECIPROCRITY (that is, I once saved your life during the war, now is the time you repay my kind action) to remind Bob Wallace (Bing Crosby).