A (Add an Aggregate step to the group and add the SUM..); C (Add an Aggregate Group to the end of the Expression Set). This maps the scenario to the Salesforce-native capability that owns the requirement.

The Core Concept Explained: Business Rules Engine externalizes decision logic from custom code. Expression Sets orchestrate calculations and can call decision assets so policy thresholds remain maintainable by authorized business users.

Step-by-Step Technical Analysis: Identify the inputs that drive the decision, declare them as Expression Set resources, and use Decision Matrices or Decision Tables where business users must maintain rule thresholds. Execute representative samples before activation, then confirm the output and Decision Explainer history match the expected eligibility, fee, or prioritization result. Use an Aggregate Group in the Expression Set and add an Aggregate step with the SUM function.

Why the Incorrect Options Are Wrong: B adds a custom build path and increases lifecycle, security, and upgrade effort when a packaged or declarative capability already supports the requirement. D adds a custom build path and increases lifecycle, security, and upgrade effort when a packaged or declarative capability already supports the requirement.