The Service

From Twitter

A. Was your dad in the service?
B. Yes–in __ navy.
A. And yours?
B. __ army.
A. Where was he stationed?
B. Alaska. My uncle was in two branches: __ Navy in world war II and the army in during the Korean war.
A. Really?
B. Yeah, but he bailed before Vietnam.

* a
* that
*the

Answers And Explanation
The answer in both cases is ‘the,’ because this is the way we talk: a special, common thing–known to everyone, such as the branches of the military, take the definite article ‘the,’ for that special quality and because of their being so common, but most importantly because they are singular countable entities.