Step 1: Break down the name.
Khalsa is a term tied directly to Sikh religious identity, so the society's work is almost certainly linked to Sikh reform.
Step 2: Place the Sikh reform movement.
Sikh religious and social reform activity, including printed tracts spreading its teachings, was centred in Punjab.
Step 3: Rule out the other regions.
Gujarat, Bengal, and Maharashtra each had their own print and reform traditions, but none of them housed the Khalsa Tract Society.
Step 4: Conclude.
Since the society's base was Punjab, the correct answer is option (D).