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!
=(Happy)&(Birthday) this is how it would be formatted
Negotiation is one possible answer to this question, I believe, though there are other words which mean similar things that could also suffice. For example, compromise is very similar in meaning, though this specifically means each person is conceding something to find an agreement.
