The app builder should use Apex code to update a field on the parent record when a child record connected via lookup is deleted. Apex code is a programming language that allows developers to execute complex logic on the Salesforce platform. Apex code can be triggered by certain events, such as insert, update, delete, or undelete of records. In this case, the app builder can write an Apex trigger that runs after a child record is deleted and updates a field on the parent record accordingly. Option A, B, and D are not automation tools that can perform this task.