Explanation:

Amazon Comprehend is the correct choice because it is AWS's natural language processing (NLP) service specifically designed for text analysis. Here's why:

1. Key phrase extraction: Amazon Comprehend can automatically identify and extract key phrases from text documents.
2. Sentiment detection: It can analyze text to determine whether the sentiment is positive, negative, neutral, or mixed.
3. Entity recognition: Amazon Comprehend can identify entities such as names, dates, locations, organizations, and more from unstructured text.
4. Scalability: It can process thousands of emails efficiently as it's a managed service that scales automatically.

Why not the other options:

• Amazon Kendra: This is an intelligent search service that helps find information across multiple data sources, not specifically for text analysis tasks like sentiment detection.
• Amazon Lex: This is a service for building conversational interfaces (chatbots) using voice and text, not for analyzing text content.
• Amazon Rekognition: This is a computer vision service for image and video analysis, not text analysis.

Amazon Comprehend is specifically designed for natural language processing tasks including the exact requirements mentioned: extracting key phrases, detecting sentiment, and identifying entities from text documents like customer emails.