a "naturalized" u. s. citizen

a served in the military for five years
b won citizenship in a contest
c was born in the united states
d completed a citizenship process

D: completed a citizenship process

After serving in the military, even for 5 years, you still have to apply for naturalization and get approved in order to naturalize through military work. You must also meet a lot of requirements beyond your military service to be eligible.
There is no citizenship contest.
Being born in the United States automatically makes you a citizen, you do not need to be naturalized.
D is the only answer that upon completion makes you a naturalized citizen because you have gone through the process of naturalization.