Which of the following is NOT a feature of Amazon Lex?

Amazon Lex is a service for building conversational interfaces using voice and text. Among its core features are conversational AI, natural language understanding (NLU), and automatic speech recognition (ASR). These functionalities allow developers to create applications that can engage users in a natural, conversational manner, recognizing spoken language and interpreting its meaning.

Conversational AI refers to the overall capability of Lex to facilitate engaging, human-like interactions. Natural language understanding enables the service to comprehend user inputs accurately, allowing it to interpret commands and intents. Automatic speech recognition converts spoken words into text, which is essential for processing voice commands effectively.

While text-to-speech is a valuable feature in other AWS services, such as Amazon Polly, it is not a capability inherent to Amazon Lex itself. Lex does not perform text-to-speech transformations; it primarily focuses on understanding and processing conversational input rather than synthesizing voice output. This distinction clarifies that text-to-speech is not part of the Amazon Lex offering, making it the correct answer to the question.