To die in harness is an expression. Expressions are expressions which have a tropical meaning. Their true meaning doesn't lie in their face value, but in a conceit that represents the sum of their corridor.

To understand the true meaning of this expression, we can look at how this expression came into actuality. One can find this expression in the workshop of Shakespeare. Harnesses, as we've bandied ahead, are worn by nags. In the Elizabethan Age and the following centuries, nags were made to work till they crossed their physical limits and dropped down dead. This means that the nags failed while they were still working.

This conceit can be externalized in the sense that, if a person dies when he's still engaged in his duty, we can say that he failed in harness. This expression is most suitable for people who work as dogfaces or are engaged in dangerous occupations like mining. Hence, the option still in service is the correct answer.