Answer: (c) The law of love
Read Luke 6: 1-5
Note: by identifying Himself as the Lord of the Sabbath, Jesus made it clear to all that as the Creator and Law Maker He would not break His own Law or be subject to the laws of men. By mentioning the time David broke a ceremonial law, because he and his men were hungry, Jesus was referring to a higher principle, the law of love.
Note: the Gospel of Matthew (12:1-8) gives a similar account to Luke 6:1-5. Matthew's account includes the words, “I desire mercy, not sacrifice” (v. 7). Speaking about this conflict with the Pharisees G. Jerome Albrecht and Michael J. Albrecht, in The People’s Bible Commentary, page 173, tell their readers:
Jesus had no difficulty in refuting the charge of the Pharisees. He did not even bother to remind them that the disciples had not broken a God-given law, but only a man-made regulation that really had no validity. He responded on a higher level, pointing out that the law of love might at times violate ceremonial laws without being guilty of sin. As a matter of fact, there are times when it would be wrong not to violate the letter of some ceremonial laws.