The app builder should provision a Partial Data sandbox to the developer. A Partial Data sandbox can have up to 5 GB of data, which meets the requirement of having at least half a gigabyte of data. A Partial Data sandbox can also be refreshed every 5 days, which meets the requirement of refreshing after each three-day sprint. Option A is incorrect because a Developer sandbox can only have up to 200 MB of data, which does not meet the requirement. Option B is incorrect because a Full Copy sandbox can only be refreshed every 29 days, which does not meet the requirement. Option C is incorrect because a Developer Pro sandbox can only have up to 1 GB of data, which may not be enough for the requirement.

The two reasons that could be preventing this relationship conversion are that there are already two master-detail relationships on the Invoice Line Item and that Invoice Line Item records exist without having the Invoice lookup field populated. A custom object can have up to two master-detail relationships, so adding another one would exceed the limit. Also, a master-detail relationship requires that the detail records have a value in the master lookup field, so any records with a blank Invoice lookup field would prevent the conversion.

The validation rule should check if the user role is not Marketing, and if the Retail Opt In field has been changed to Yes. The ISCHANGED function returns true if the field value has been changed, and the ISPICKVAL function returns true if the field value matches a specified picklist value.

Reference:
Salesforce Help - External ID

Create a master-detail relationship between the parent and child object with a roll-up summary field that filters on the status field is the recommended method for the app builder to achieve the request of rolling up sales metrics to an opportunity for only Detailed.Sales__c records that have their picklist status set to Active. According to the Salesforce documentation, "Roll-up summary fields calculate values from related records, such as those in a master-detail relationship." A roll-up summary field can filter on a field value of the child records and sum up only those records that match the criteria. Utilize the AppExchange to download a third-party application that can roll up the sales dollars with the appropriate filter, create a lookup relationship between the parent and child object with a roll-up summary field that filters on the status field, and utilize Apex code to roll up the desired amounts are not valid or necessary methods for this request.