Step 1: Identify the context.
The sentence expresses a strong obligation.
Step 2: Rule of modal usage.
"Must" is used for strong necessity or moral duty, which fits "keep our promises."
Step 3: Eliminate options.
(A) Ought – also possible but weaker, less forceful than "must."
(B) Need – implies requirement but not as strong.
(D) Could – expresses ability, not obligation.