Ursa Major Solar has a lookup relationship between a custom Galaxy _c object and a custom Star_c object. An app builder wants to create a roll-up…

Roll-up summary fields are a specific feature of master-detail relationships in Salesforce. They aggregate data from child (detail) records and display the result on the parent (master) record. The current configuration uses a lookup relationship, which is a more loosely coupled connection and does not support roll-up summary fields. To create a field that counts the number of Starc records on the Galaxyc object, the relationship must first be converted from a lookup to a master-detail relationship.

A. Formula fields on a parent object cannot aggregate or count child records; they operate on data within a single record or pull data from a direct parent.

C. A formula field on the child (Starc) object would calculate a value for that specific record, not an aggregate count for the parent Galaxyc record.

D. The creation of a roll-up summary field is fundamentally impossible on an object that is part of a lookup relationship; the relationship type must be master-detail.

1. Salesforce Help, "Roll-Up Summary Field": "You can create a roll-up summary field to display a value in a master record based on the values of fields in a detail record. The detail record must be related to the master through a master-detail relationship." This directly confirms that a master-detail relationship is a prerequisite.

2. Salesforce Help, "Considerations for Relationships": In the comparison table between relationship types, the "Roll-Up Summary Fields" feature is listed as available only for the master object in a master-detail relationship, explicitly excluding lookup relationships from this capability.

3. Salesforce Help, "Convert Relationships": This document outlines the procedure for changing a relationship type. It states, "You can convert a lookup relationship to a master-detail relationship, but only if the lookup field in all the records contains a value." This confirms that the action described in the correct answer is a valid and necessary step.