AWS Certified Cloud Practitioner

Explanation:

Amazon Lex is the correct choice because it is an AWS service specifically designed for building conversational interfaces using voice and text. Here's why:

1. Amazon Lex is a service for building conversational interfaces into any application using voice and text. It provides the deep learning functionalities of automatic speech recognition (ASR) for converting speech to text, and natural language understanding (NLU) to recognize the intent of the text.

2. Amazon Polly is a text-to-speech service that converts text into lifelike speech, but it doesn't handle conversational interfaces or natural language understanding.

3. Amazon Transcribe is an automatic speech recognition service that converts speech to text, but it doesn't provide conversational interface capabilities.

4. Amazon Translate is a neural machine translation service that translates text between languages, not for building conversational interfaces.

For a virtual teaching assistant that needs both voice and text conversational capabilities, Amazon Lex is the appropriate AWS service as it can handle:

• Voice interactions through ASR
• Text-based conversations
• Natural language understanding to interpret user intent
• Dialog management to maintain conversation flow
• Integration with other AWS services for backend processing