Concept:The required word is part of a phrasal verb meaning to reconcile after a disagreement.
Explanation:The sentence describes two friends becoming friendly with each other again after an argument.
No math or chemical formulas are needed here.
Consider each option:
A. off: "made off" means ran away or escaped, which does not fit friendship.
B. do: "made do" means managed with what is available, which is not suitable.
C. away: "made away" means departed or stole something, so it is wrong.
D. up: "made up" means forgave each other and became friends again.
This exactly matches the situation in the sentence.
Answer:D. up