The correct answer is: C. die while still working.
To die in harness means to die while still working, especially in the armed forces. It is considered a great privilege because it shows that the person died doing what they loved and were good at. It also shows that they died serving their country or their cause.
Option A is incorrect because it is not specific to the armed forces. People can die on horseback for many reasons, not just in the armed forces.
Option B is incorrect because it is not specific to working. People can die in the battlefield for many reasons, not just while working.
Option D is incorrect because it is too general. There are many ways to die with honor, not just while working.