Answer:
13:a. 15:c. 14:Unknown answer
Hello there!
They already gave you the answer just by saying "while serving his or her nation". So, the only way you can serve your nation is when you join the military.
So, the correct missing word is Joining the military.
I hope this helps!
It could be interchangeable with the word program
If you are just starting go on khan academy and go to the computing section and they will start you out on the basics on code devoloping.