Opportunities are in a master-detail-like relationship with Accounts (standard lookup behaves with special roll-up capabilities). Salesforce provides a standard roll-up summary feature on the Account object to count related Opportunities.
* Why B (Roll-up Summary) is correct:Exact Extract:"Use a roll-up summary field to automatically display a value from related detail records on a master record. You can count related records, or calculate the sum, min, or max of a field on those records." - Salesforce Help | Roll-Up Summary FieldsOn Account, you can create a roll-up summary field to COUNT related Opportunities.
Why B (Roll-up Summary) is correct:Exact Extract:"Use a roll-up summary field to automatically display a value from related detail records on a master record. You can count related records, or calculate the sum, min, or max of a field on those records." - Salesforce Help | Roll-Up Summary FieldsOn Account, you can create a roll-up summary field to COUNT related Opportunities.
* Why not A:Formula fields cannot count related records.
* Why not C:AppExchange is not required; Salesforce provides native functionality.
* Why not D:A lookup only links records-it does not perform aggregation.
References:
Salesforce Help | Roll-Up Summary Fields
Salesforce Platform App Builder Exam Guide | Data Modeling and Management