Explanation:

To integrate a client's existing phone system with Salesforce, the consultant should use Service Cloud Call Center. This feature is the standard Salesforce tool for integrating third-party Computer Telephony Integration (CTI) systems.

A. Service Cloud Call Center: This feature provides the framework for integrating a client's telephony system with Salesforce. It allows a developer or a partner to use the Open CTI API to build a softphone that agents can use directly within the Service Console. The call center configuration file defines the layout and functionality of the softphone, which can then be assigned to specific user profiles. This enables features like screen pops for incoming calls, click-to-dial functionality, and automatic call logging, all of which are essential for a CTI integration.

B. Lightning Dialer: This is an add-on feature from Salesforce that provides a native, out-of-the-box phone solution. It's a great option for clients who don't have an existing phone system and want a simple, integrated solution. However, since the client in this scenario already has a phone system that needs to be integrated, Lightning Dialer is not the correct choice.

C. Service Cloud Softphone Layout: The softphone layout is a component of the Call Center feature. It's the visual interface that agents see and interact with to manage calls. While the consultant will need to configure the softphone layout, it is not the overarching solution for the integration itself. The softphone layout is defined within the Call Center settings, making Service Cloud Call Center the correct parent solution.