validation Rule

Explanation:

A validation rule is the most appropriate solution because it: 1) Enforces the "either/or" requirement precisely (phone OR email); 2) Works consistently across all interfaces (UI, API); 3) Provides immediate user feedback; and 4) Requires no ongoing maintenance. Required fields (B) would mandate both fields instead of either. Apex (C) adds unnecessary complexity for this simple rule. Process Builder (D) isn't designed for real-time validation. The validation rule would use logic like: "AND(ISBLANK(Phone), ISBLANK(Email))" to check both fields are not empty, displaying a custom error message when violated. This approach: 1) Prevents record save unless the condition is met; 2) Clearly communicates requirements to users; 3) Executes efficiently as part of the save operation; and 4) Can be easily modified if requirements change. Validation rules are the standard Salesforce solution for field-level data quality rules, providing robust enforcement without coding. The declarative nature also makes the requirement visible to administrators and auditors reviewing the org configuration.