The Data Import Wizard is the most appropriate tool for this scenario. It is an in-browser tool designed for end-users and administrators to perform simple data imports of less than 50,000 records. It supports common standard objects, including Accounts and Contacts, and provides a user-friendly, step-by-step interface. Given the small volume of 300 records and the standard objects involved, the Data Import Wizard is the most efficient and straightforward choice.

A. Dataloader.io: This is a more powerful, web-based data loading tool, but it is generally used for more complex integrations or larger data volumes than what is required here.

B. Manual Import: This refers to entering records one by one, which is not a bulk loading tool and is highly inefficient and impractical for 300 records.

D. Data Loader: This is a client application that requires installation and is intended for large-scale data operations (50,000 to 5 million records) or for objects not supported by the wizard.

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1. Salesforce Help Documentation, "Which Data Import Tool Should I Use?": This official guide explicitly recommends the Data Import Wizard for loads of less than 50,000 records. It states, "If you need to load less than 50,000 records, use the Data Import Wizard." The scenario of 300 records falls well within this limit.

2. Salesforce Help Documentation, "Import Data with the Data Import Wizard": This document details the wizard's capabilities, noting it supports "accounts, contacts, leads, solutions, campaign members, person accounts, and custom objects." This confirms its suitability for the specified record types (Accounts and Contacts).

3. Salesforce Trailhead, "Data Management" Module, "Import Data" Unit: This official training module positions the Data Import Wizard as the tool for "simple data imports" for a limited number of records. It contrasts this with Data Loader, which is recommended for "complex data imports" or large volumes. The scenario aligns with the definition of a simple import.