A cross-object formula field is the correct solution to display data from a related parent record (Account) on a child record (Opportunity). By creating a formula field on the Opportunity object that references the AccountRegionc field on the Account (e.g., Account.AccountRegionc), the value is dynamically pulled and displayed. This ensures the data is always current and visible on the Opportunity record without requiring data duplication or complex automation. This is a standard, declarative feature designed for this exact use case.

A. Lookup field: A lookup field is used to create a relationship between two objects, not to display a field value from an existing related record.

B. Field-level security: This feature controls the visibility and editability of a field for different user profiles; it does not create the field or pull data from another object.

C. Field history tracking: This is an auditing tool used to log changes to a field's value over time, not for displaying data across related records.

1. Salesforce Help Documentation, "Create a Cross-Object Formula Field": This document explicitly states, "A cross-object formula is a formula that spans two related objects and references merge fields on those objects... You can reference merge fields from a parent object ('master' object) to display on a child object ('detail' object)." The example provided in the question fits this definition perfectly.

2. Trailhead, "Formulas and Validations" Module, "Use Cross-Object Formulas" Unit: This educational unit explains, "Cross-object formulas are formulas that span two or more objects by referencing merge fields on related objects." It provides a direct analogy: creating a formula on a Contact to display the related Account's phone number using the syntax Account.Phone. This is the same principle required to solve the problem.

3. Salesforce Help Documentation, "Formula Operators and Functions": Under the section for inserting fields, the documentation details how to access fields from related objects via lookup relationships. For a standard object like Opportunity looking up to Account, the syntax Account.FieldName is the specified method.