Which of the following commands converts spaces in a file to tab characters and prints...

The unexpand command converts spaces in a file to tab characters and prints the result to standard output. It can take an option -t to specify the number of spaces per tab, otherwise it uses the default value of 8. Theunexpand command is the opposite of the expand command, which converts tabs to spaces. Both commands are useful for formatting text files according to different coding standards or preferences. References:

Unexpand - Convert Spaces to Tabs on Linux Command Line

Bash Tips: Convert Tabs To Spaces In a File