Explanation:

Amazon Polly is the correct AWS service for converting text into natural-sounding speech. Here's why:

1. Amazon Polly is AWS's Text-to-Speech (TTS) service that converts text into lifelike speech using advanced deep learning technologies.

2. Key features relevant to the question:

   • Supports multiple languages and voices
   • Provides natural-sounding speech output
   • Can be integrated into mobile applications
   • Offers neural text-to-speech (NTTS) for more natural speech patterns

3. Why other options are incorrect:

   • Amazon Transcribe (Option A): This is a speech-to-text service, converting audio to text, not text to speech.
   • Amazon Translate (Option C): This is a machine translation service for translating text between languages, not for speech synthesis.
   • Amazon Comprehend (Option D): This is a natural language processing (NLP) service for extracting insights and relationships from text, not for speech generation.

For a mobile app that needs to read out user notifications in natural-sounding speech across multiple languages, Amazon Polly is the ideal AWS service choice.