Which AWS service will meet this requirement?

Explanations:

Amazon Comprehend is a natural language processing service that provides insights into text, such as sentiment analysis and entity recognition. It is not designed for video or audio file conversion.

Amazon Rekognition is an image and video analysis service that can identify objects, people, text, scenes, and activities in images and videos. It does not provide functionalities for converting audio or video formats.

Amazon Elastic Transcoder is specifically designed for converting media files, including audio and video, into formats suitable for playback on various devices, including smartphones. It supports a wide range of codecs and output formats.

Amazon Polly is a text-to-speech service that converts written text into spoken words. While it deals with audio, it does not perform media file conversion or transcoding for video files.