We’ll start with the end result and work backwards to identify necessary conditions for the question. We’ll then infer the question from these conditions.

The end result is, of course, that we know which Regiment we’re visiting: Jat or Sikh LI.

One step before knowing which Regiment we’re in was to get a “Yes” or “No” answer.

But if we knew which Regiment we were visiting just by getting a “Yes” or “No” answer it would mean that we would have to construct the question is such a way that one response (”Yes”, for example) would be associated with the truth tellers’ Regiment (given only in that Regiment), and the other response (”No”, in this case) would be associated with the liars’ Regiment (again, given only in that Regiment).

Now, let’s suppose that we’re in the truth tellers’ Regiment.

If we happen to meet a Soldier from the Regiment (a truth teller) and ask our magic question, the answer must be “Yes”.

But if we happen to meet a liar in the truth tellers’ Regiment, the answer must also be “Yes”.

If both a liar and a truth teller provided the same response for the same question, it would mean that the question must relate to the soldier being asked. This is the only way the same question will be interpreted differently by different kinds of people.

Let’s now suppose that we meet a truth teller in the liars’ Regiment. We would ask the same magic question, but now we must get the answer, “No”.

This can happen only if the question is related to the ‘Regiment’. It is the only way the same question will be interpreted differently by the same person in different Regiment.

So we know the question relates to the Regiment and to the soldier.

Therefore, it is probably about the RELATIONSHIP between the soldierand the Regiment.

Now, equipped with all the information about the question, it’s time to try a question.

What about, “Is this your Regiment?”

Let’s see -

If we ask a truth teller in the truth tellers’ Regiment, we’d get a “Yes” answer. If we ask a liar in the truth tellers’ village, we’d also get a “Yes” answer. (because he’d lie). If we ask a truth teller in the liars’ village, we’d get a “No” answer. If we ask a liar in the liars’ village we’d also get a “No” answer (because he’d lie).