Which mode allows you to read from a file without altering its contents?

The mode that allows you to read from a file without altering its contents is denoted by 'r'. When a file is opened in this mode, it is opened for reading only, meaning that no changes can be made to the existing content of the file. This ensures data integrity, as the original contents remain unaffected during the reading process.

In contrast, the 'a' mode is used for appending new data to the end of an existing file, which does modify the file. The 'w' mode opens a file for writing and will overwrite any existing data, completely altering its contents. The 'rw' mode implies a combination of read and write permissions, allowing modifications to occur, thus potentially changing the original data in the file.

With 'r', you can examine the file and retrieve data without any risk of unintentional modification, making it the correct choice for reading without changes.