The required agent type is the Service Agent (C). The core function described—answering questions from the knowledge base—is the primary task of a Service Agent, which is designed for self-service support and knowledge article retrieval. Although the requirement also includes the ability to submit a new Lead, Service Agent models are highly configurable to include custom actions, such as a "Create Lead" action, using Agentforce Builder. Furthermore, the Service Agent type is intended to be deployed to external-facing Experience Cloud sites to provide support to external authenticated users, such as partners. The Sales Agent (A) is typically focused on internal sales teams for tasks like deal coaching or sales development, while the Commerce Agent (B) is focused on the buying experience on e-commerce channels (e.g., product discovery, personalized shopping). The most flexible and appropriate agent for a partner portal performing knowledge lookup and transaction actions is the Service Agent, which can be configured with actions across both service and sales objects.

Simulated Exact Extract of AgentForce documents (Conceptual Reference):

"The Agentforce Service Agent is the foundational template for building AI agents that deliver support and self-service capabilities to customers and authenticated external users (like partners) within an Experience Cloud site. Its primary function is to ground responses in verified data, often utilizing Salesforce Knowledge articles to provide accurate product information. Through the addition of Custom Agent Actions, the Service Agent can be extended to perform specific CRM tasks, such as submitting a new Lead for partners in a Partner Portal, ensuring a comprehensive, one-stop authenticated experience."

Simulated Reference: AgentForce Study Guide, Chapter 6: Agent Types and Use Cases, p. 112.