The app builder should create a new roll-up summary field called First Order Date, using Type MIN on the Opportunity Close Date with a filter where IsWon = TRUE. A roll-up summary field is a type of field that calculates values from related records and displays them on the parent record. In this case, the app builder can create a roll-up summary field on the Account object that calculates the minimum (earliest) value of the Opportunity Close Date field from the related opportunities where IsWon = TRUE. This will show when customers made their first purchase. Option B, C, and D are not correct ways to use a roll-up summary field to meet the requirement.

The Account record includes Invoice roll-up summary fields. This is correct because roll-up summary fields are only available on the master object in a master-detail relationship, and changing the field type to lookup would delete the roll-up summary fields

Create a validation rule on the Account object is how the app builder can keep accounts with open opportunities from being deleted. According to the Salesforce documentation, "Validation rules verify that data entered by users in records meet the standards you specify before they can save it." A validation rule can check if an account has any open opportunities and display an error message if someone tries to delete it. Create an Apex trigger on the Account object, remove the delete button on the account layout, and remove the Delete permission from the Sales Rep profile are not valid or sufficient solutions for this requirement.