Which file mode should be used to read a file without altering its contents?

To read a file without altering its contents, the appropriate file mode to use is "r mode." This mode stands for "read" and allows you to open a file for reading only. When a file is opened in this mode, you can access its data, but you cannot make any changes to it. This ensures that the original contents of the file remain intact.

In contrast, other modes, such as "w mode," open a file for writing and will truncate (empty) the file if it already exists, effectively altering its contents. "a mode" is for appending data to the end of the file, which also modifies the file, and "w+ mode" allows both writing and reading but will truncate the file upon opening, thereby modifying its contents as well. Therefore, using "r mode" is the only way to ensure that you can read a file without making any alterations to it.