The one that defines how you are required to honor an abusive parent.
HonorAgain, I reiterate: you misunderstand what it is to 'honor', I direct your attention to the definition above, particularly the bold & underlined definition. That definition is the the most applicable, though the others are too. Applying it to the commandment is to be a source of credit to your parents, the sort of person that makes people say that they did a good job raising you. (That they did or did not is no matter, the point is your character; to put it in militaristic terms, serving under a cruel and despotic commander would not alleviate me of the accountability of acting with dignity and respect to him or to others.) Nothing about 'honor' is to encourage a soft yielding-capitulation to evil.
- honesty, fairness, or integrity in one's beliefs and actions: a man of honor.
- a source of credit or distinction: to be an honor to one's family.
- high respect, as for worth, merit, or rank: to be held in honor.
- such respect manifested: a memorial in honor of the dead.
- high public esteem; fame; glory: He has earned his position of honor.
As the bold-only shows, there is a respect element. "Respect" is not a self-effacement, but to hold in some manner of esteem. To go back to the military example: even if the commander is a lying cheat there is no excuse for me to discount his authority "because he's a bad guy." To react that way would be to undermine my own dignity.