Which file types can be specified to narrow search results?

Selecting "All of the above" is appropriate because each of the mentioned file types—Word, Text, and PDF—can be specified when conducting searches to filter results based on file format. This feature is particularly useful when specific content types are needed for research or document retrieval.

Using file type specifications can significantly enhance the relevancy of search results; for instance, if a user is seeking a specific document format that aligns with their software capabilities or preferences (e.g., they might only want to access PDF files for ease of viewing on portable devices), specifying the file type ensures a more efficient search process.

In this case, since both Word documents and text files are also commonly used formats, the option to narrow search results by these types is viable, making "All of the above" the correct option.