The answer is (D) taking up arms to defend the nation exempts a person from religious reasons.
In all the other three cases anybody would be eligible to do so as a citizen.
But taking up arms to defend the nation not only needs mental courage and physical strength but also some motivation to represent the entire nation. Therefore, those taking up arms to defend their nation would be doing some service which is what religion aims at. They are both one and the same serving people in general or serving people during crisis (D). This social service by taking the arms to defend the nation coincides with the goal of Religion.