The correct option is QnAMakerDialog.

The QnAMakerDialog is the Bot Framework dialog class that is specifically built to query a question and answer knowledge base and return FAQ style answers to user queries. It wraps the integration with a QnA knowledge base so the conversational assistant can match user utterances to the best KB answer and handle follow up prompts and metadata when needed.

Be aware that the QnA Maker capability has been succeeded by Azure Cognitive Services for Language question answering in newer stacks. Newer SDKs and exams may reference the Language service Question Answering features or updated dialog wrappers rather than the legacy QnAMakerDialog.

SkillDialog is intended for invoking external skill bots and delegating parts of a conversation and not for looking up answers from an FAQ knowledge base.

Dialogflow CX is a Google Cloud conversational platform and not a Bot Framework dialog class, so it is not the correct choice when the question asks which dialog class to use in this context.

AdaptiveDialog is a general purpose Bot Framework dialog for building complex, state driven conversational flows and it does not provide the built in knowledge base lookup and answer matching that QnA Maker provides.

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