The national highways in India each have a specific number for easy identification and navigation. The question here is asking for the number of the national highway that runs from Delhi to Calcutta (now Kolkata) passing through the cities of Mathura and Varanasi. Option 1, option 3, and option 4 suggest that this route is numbered as 1, 4, and 8. However, that`s not correct. The correct answer is Option 2 which suggests the highway number for this route is 2. Indeed, National Highway 2, which has been renumbered as NH 19 in the recent numbering system, is the one that goes from Delhi to Kolkata passing through Mathura and Varanasi. So, the correct answer is really 2.