The correct option is Azure Bot Service.

Azure Bot Service is the platform that provides hosting and orchestration for conversational applications and it integrates with the Bot Framework SDK and Composer to implement dialog management, state handling, and channel connectors. It also connects to Cognitive Services for language and speech features, but the Bot Service is the primary runtime and management layer for building, deploying, and scaling conversational agents.

Azure Cognitive Search is focused on indexing and retrieving content and it is useful for search scenarios but it does not provide dialog orchestration or bot hosting. You could use it to surface search results inside a bot, but it is not the service that implements the conversational agent.

Azure Blob Storage is an object storage service for storing files and binary data and it does not include dialog logic, connectors, or a runtime for bots. It can store assets or logs for a bot but it is not the bot platform.

Azure Virtual Network provides networking and isolation for resources in Azure and it is not a service for building or managing conversational agents. It can secure deployments but it does not handle dialogs or user interactions.

Azure Cognitive Services offers AI building blocks for vision, speech, and language and it can supply natural language understanding and speech capabilities to a bot. It is not itself the dialog orchestration and hosting platform that Azure Bot Service provides, so it is not the correct answer.

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