B. Language Understanding
Explanation:
To meet the requirement that the chatbot must support English, French, and Portuguese and provide other response options when the response confidence score is low, the Language Understanding component (often referred to as LUIS - Language Understanding Intelligent Service) is needed. LUIS allows the bot to understand the user's intent in multiple languages and can help to provide alternative responses.
Microsoft Translator could translate between languages, but it does not handle intent understanding, so it's not the best choice here. Orchestrator is more about managing multiple LUIS and QnA Maker models, which isn't specified in the requirements. Chatdown is a tool for creating mockups of chatbot conversations but doesn't provide the functionality needed to meet the requirements.