To prevent duplicate lead creation during an import of 400 records, two primary methods are available.

First, configuring Salesforce's native Duplicate Management feature is a robust, org-wide solution. This involves creating a Matching Rule to define the criteria for identifying a duplicate (e.g., same email address) and a corresponding Duplicate Rule set to 'Block' the creation of records that meet this criteria. This prevents duplicates from all sources, including data imports.

Second, for a data load of this size (under 50,000 records), the Data Import Wizard is the appropriate tool. The wizard has built-in functionality that allows the user to select a field to match against existing records (e.g., 'Match Lead by: Email'). This feature prevents the creation of new records if a matching lead is found.

C. Data Loader's update function can only modify existing records identified by a Salesforce ID; it cannot create new leads from an external list. The upsert function would be needed for that.

D. This option uses imprecise terminology. "Duplicate Matching" is not a single setting, and "Lead-to-Lead scenario" is not standard Salesforce terminology for configuring Duplicate and Matching Rules.

1. Salesforce Help Documentation, "Manage Duplicate Records": This document explains the core components of Duplicate Management. It states, "A duplicate rule is a rule that determines what happens when a user views a record with duplicates or starts creating a duplicate record. Salesforce provides standard duplicate rules for business and person accounts, contacts, and leads... You can also create your own duplicate rules." This supports option A.

2. Salesforce Help Documentation, "Set Up Duplicate Management": This guide details the process: "A matching rule compares records and identifies duplicates. A duplicate rule determines what to do with duplicates." It specifies that a duplicate rule's action can be set to "Block" or "Allow". This further validates option A.

3. Salesforce Help Documentation, "Import Data with the Data Import Wizard": This guide describes the wizard's capabilities. In the section on importing leads, it details the step for "Choose how to handle duplicate records," where you can select a matching type like "Match by email" to "prevent duplicates from being created." This directly supports option B.

4. Salesforce Developer Documentation, "Data Loader Guide", Insert, Update, or Delete Data Using Data Loader: This guide defines the update operation: "Use the Update button to make changes to existing records. The records are identified by their Salesforce IDs." This confirms that the update function is inappropriate for creating new records, invalidating option C.