The requirement is to automate a business process that updates related child records (Opportunity Products) when a parent record (Opportunity) meets specific criteria (Stage is 'Won'). Flow Builder is the designated Salesforce tool for complex, record-triggered automation. An administrator can create a record-triggered flow that initiates when an Opportunity record is updated to 'Won'. This flow can then get all related Opportunity Product records and loop through them to update their discount fields with the value from the parent Opportunity, completely automating the manual task.

B. Approval Process: This tool is designed to automate the formal approval of records. It is not the correct tool for triggering data updates on related records based on a field change.

C. Prebuilt Macro: Macros are used to automate repetitive tasks performed by a user in the UI. They must be manually run by a user and cannot be automatically triggered by a record update.

D. Formula field: A formula field calculates and displays a value in real-time based on other fields. It is read-only and cannot be used to update the value of another field.

1. Flow Builder:

Salesforce Help, "Flow Builder". The documentation states, "Flow Builder is the declarative interface used to build individual flows. Flow Builder can be used to build code-like logic without using a programming language." It is the primary tool for record-triggered automation that involves related records.

Salesforce Help, "Record-Triggered Flow". This section details how flows can be configured to run "when a record is created, updated, or deleted." This directly addresses the trigger requirement (Opportunity is updated to 'Won').

2. Approval Process:

Salesforce Help, "What Is an Approval Process?". The documentation defines it as a tool that "automates how records are approved in Salesforce... An approval process specifies the steps that are necessary for a record to be approved and who approves it at each step." This confirms it is for approvals, not general data updates.

3. Macros:

Salesforce Help, "Salesforce Macros". The documentation explains, "Macros help your team save time... A macro is a set of instructions that tells the system how to complete a task... Agents run the macro to complete the task." This highlights that macros are user-initiated.

4. Formula Fields:

Salesforce Help, "Formula Fields". The documentation states, "A formula is an algorithm that derives its value from other fields, expressions, or values... Formula fields are read-only." This confirms they cannot be used to write or update data in other fields.