Why a Trigger on the Child Object?
Since the relationship is not master-detail, a trigger on the child object can perform aggregate calculations and update the parent Opportunity.
Why Not Other Options?
A: Tree sorting is unnecessary and unrelated to roll-up summaries.
B and D: Neither Streaming API nor Metadata API is suitable for real-time roll-up calculations.
References:Custom Roll-Up Summaries:https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta
/apexcode/apex_triggers_best_practices.htm