Which regular expression turns the input stream into the output stream with .bak extension?

The correct choice effectively modifies the input stream by replacing a filename that ends with .txt to have a new extension of .bak, ensuring that only the .txt at the end of the string is targeted.

In regular expressions, the dollar sign ($) is a special character that signifies the end of a line or a string. By using txt$, this regular expression specifically looks for any line ending with txt. The replacement part of the expression, bak.txt, takes the matched text and changes it so that the filename now ends with .bak instead of .txt. This would convert a file named example.txt into example.bak.

This choice clearly indicates the desired file extension transformation based on the structure of filenames that are being processed, which makes it concise and appropriate for the task.