The correct answer is option 2, NH-7. The National Highway 7 (NH-7) is the longest National Highway in India, stretching from Varanasi in Uttar Pradesh to Kanyakumari in Tamil Nadu. It covers a distance of approximately 2,369 kilometers. NH-2, also known as the Delhi-Kolkata Road, is the second longest National Highway in India, running from Delhi to Kolkata and spanning a distance of about 1,468 kilometers. NH-8, also known as the Delhi-Mumbai Road, is the third longest National Highway in India, covering a distance of around 1,296 kilometers. NH-31, on the other hand, connects Barhi in Jharkhand to Guwahati in Assam, with a length of approximately 1,125 kilometers. Therefore, NH-7 is the correct answer as it is the longest National Highway in India, surpassing the other options in terms of total distance covered.