The scenario describes a need for a single Contact record (the contractor) to be associated with multiple Account records (the agencies) over time. The standard Salesforce feature "Contacts to Multiple Accounts" is designed for this exact purpose. It allows users to create direct relationships between one contact and multiple accounts, designating one as primary. This maintains a single, historically accurate record for the contractor while tracking their affiliations with different agencies, fulfilling the core business requirement without creating duplicate data.

A. Use a partner community to track the Contacts.

A Partner Community is a collaboration platform for external users. It does not fundamentally change the data model relationship between Contacts and Accounts.

B. Create a new Contact record for each agency.

This would create duplicate records for the same person, which contradicts the goal of maintaining a single, historically accurate record for the contractor.

C. Create a Junction object to track many-to-many relationship.

While a custom junction object can create a many-to-many relationship, it is a custom solution. The standard "Contacts to Multiple Accounts" feature is the preferred, out-of-the-box method.

1. Salesforce Help, "Contacts to Multiple Accounts": This document outlines the feature's purpose and functionality. It states, "Contacts to Multiple Accounts lets you relate a single contact to multiple accounts so you can easily track the relationships between people and businesses."

Source: Salesforce Help, Article: "Contacts to Multiple Accounts".

2. Salesforce Help, "Set Up Contacts to Multiple Accounts": This guide details the setup process and considerations. It confirms that this feature is the intended solution for relating contacts to more than one account.

Source: Salesforce Help, Article: "Set Up Contacts to Multiple Accounts".

3. Salesforce Trailhead, "Data Modeling" Module, "Object Relationships" Unit: This module explains object relationships. While it discusses junction objects for many-to-many relationships, it also highlights standard features like "Contacts to Multiple Accounts" as purpose-built solutions for specific common scenarios, which are preferable to custom builds.

Source: Salesforce Trailhead, Trail: "Admin Beginner", Module: "Data Modeling", Unit: "Understand Object Relationships".