The most effective and recommended solution is to use Salesforce's native Duplicate Management feature. This declarative tool allows an administrator to define what constitutes a duplicate record through matching rules (e.g., matching on email address and last name). The administrator can then create duplicate rules that specify the action to take when a user attempts to create a duplicate, such as alerting the user, blocking the creation, or logging a report. This directly addresses the requirement to prevent duplicate records from being created systemically, without resorting to custom code or insecure permission changes.

B. Granting "View All Data" is a significant security risk and does not enforce duplicate prevention; it relies on inconsistent user behavior to manually search for records.

C. Implementing Apex triggers is a custom code solution. It is Salesforce best practice to use standard, declarative features like Duplicate Management before developing custom code.

1. Salesforce Help Documentation: Set Up Duplicate Management. This document outlines the process: "To start using duplicate management, create matching rules and duplicate rules... A matching rule determines how records are identified as duplicates. A duplicate rule determines what to do when duplicates are identified."

Source: Salesforce Help, "Set Up Duplicate Management," accessed via Salesforce official documentation portal.

2. Salesforce Trailhead: Duplicate Management Module, "Get Started with Duplicate Management" Unit. This module explicitly states, "With Duplicate Management, you can control whether and when users can create duplicate records in Salesforce." It details how matching rules identify duplicates and duplicate rules handle them, confirming this is the intended tool for the scenario.

Source: Salesforce Trailhead, Module: Duplicate Management, Unit: Get Started with Duplicate Management.

3. Salesforce Architects Documentation: Record-Triggered Automation Guide. This guide reinforces the "clicks-not-code" principle, stating, "Before you write any code, see if you can solve your business problem using one of the declarative automation tools." This supports choosing Duplicate Management (declarative) over an Apex trigger (code).

Source: Salesforce Architects, "Record-Triggered Automation," Decision Guide section.