Extracting repeated code into a function usually makes a program longer, not shorter, once the definition and calls are counted. Why is that a good trade?

B. One place where the logic lives, so a fix is made once.

Duplicated logic means duplicated bugs, and the failure mode is fixing four of five copies, worse than not noticing, because the bug now appears intermittently. The name does real work too: is_valid_email(s) tells a reader what the code accomplishes without their reading how. A call costs a little, so modularity is bought at a small runtime price worth paying.