An app builder is tasked with adding key performance indicators on client pages. They want to see a summary of the number of open Opportunities and…

Roll-up summary fields are created on the master object of a master-detail relationship to aggregate data from related detail records. In this scenario, Account is the master (parent) object, and Opportunity is the detail (child) object. To create a summary of Opportunity records (e.g., count of open or won opportunities), the app builder must define the roll-up summary fields on the Account object. This is done within the Object Manager by navigating to the Account object's "Fields & Relationships" section.

A. Lightning App Builder: This tool is used to design the user interface for Lightning pages by arranging components and fields. You can place an existing roll-up field on the page layout here, but you cannot create the field itself.

C. Lightning Object Creator: This tool is used for creating new custom objects from external data sources like spreadsheets. It is not used for modifying existing standard objects such as Account.

D. Opportunity Object: The roll-up summary field is always created on the parent object (Account) to summarize data from the child object (Opportunity), not the other way around.

1. Salesforce Help, "Create a Roll-Up Summary Field": This document explicitly states, "You can create roll-up summary fields on any custom object that is on the master side of a master-detail relationship" and "on standard objects that are on the master side of a master-detail relationship with a custom object." It also applies to certain standard-to-standard relationships like Account-Opportunity. The creation steps begin with navigating to the setup for the master object.

2. Salesforce Help, "Roll-Up Summary Field": Under the "Notes on Roll-Up Summary Fields" section, it clarifies, "A roll-up summary field calculates values from related records, such as those in a related list." This confirms the field resides on the parent object that has the related list.

3. Salesforce Trailhead, "Create Object Relationships": In the "Create a Roll-Up Summary Field" unit, the first step of the hands-on challenge is: "From the Object Manager, find and select the master object," reinforcing that the field is built on the parent object (Account in this case).