To index a collection of text documents for search in a mobile application, which Azure service should be used?

Azure Cognitive Search is specifically designed to index and search content, making it the most suitable service for indexing a collection of text documents for search functionalities in a mobile application. This service allows users to create full-text search capabilities and manage searchable fields, filters, and faceting.

With Azure Cognitive Search, you can crawl and index various content types, including documents in different formats, making it easier to retrieve relevant information efficiently. It supports functionalities like keyword search, text analysis, scoring profiles, and synonym maps, which enhance the search experience for users.

In contrast, the Language cognitive service focuses on natural language processing tasks like sentiment analysis and key phrase extraction but does not provide full-fledged indexing and search capabilities. Azure Bot Service is geared towards developing conversational interfaces and chatbots, not document indexing. Azure Data Factory serves as a data integration service, primarily for orchestrating data workflows but does not contribute directly to the indexing of text documents for search purposes.