To paste the text into another document and ensure that the capitalization is removed, Chris has to highlight the whole text, copy it and then paste it into Word. After this, he has to go to the Home tab and click the change case button which will show a dropdown menu that has different options that he can choose from like lowercase and sentence case. When he clicks on the option he prefers, the text will be modified.
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.