Explanation:

The History utility is a standard Salesforce Lightning Console feature designed specifically for this purpose. It provides a list of recently visited records, including tabs and subtabs. By including this utility in the console app, service agents can easily access and share links to the specific cases, contacts, or service contracts they have recently opened.

How it Works:
The History utility automatically tracks the user's navigation within the console. When an agent opens a case subtab, that subtab is added to the history list. The agent can then click on the History utility icon to see a list of recent items and simply copy the URL to share with another user. This is the most efficient and direct way to meet the requirement of sharing links to recently opened subtabs.

Why the other options are incorrect:
A. Add the Account object to Recent Items utility: The Recent Items utility shows a list of recently accessed records, not a list of recently opened tabs or subtabs. While it might show the Account record, it won't show the specific subtabs for the Case, Contact, and Service Contract that the agent needs to share. The History utility is more granular and tracks the user's specific console navigation.
C. Mention the case number in a Chatter group: This is a manual and inefficient process. While an agent could manually copy and paste the case number into Chatter, it doesn't directly provide a shareable link to the specific subtab the agent was viewing. This method would require the other user to then manually search for and open the record, which is not the streamlined collaboration process the requirement seeks. The History utility provides a direct link, making collaboration much faster.

References:
Salesforce Help Documentation: "History Utility Item." This document explains how the History utility works and its purpose within the Lightning Console.
Salesforce Trailhead: "Lightning Service Console Basics" module. This module covers the various components of the Service Console, including the utility bar and its standard components like History.