Since your running is high, it would be a choice between CNG and diesel. The Tata Nexon and Mahindra XUV 3XO are both available with a diesel engine, but with a less-impressive AMT gearbox.
The Hyundai Venue and Kia Sonet also get a diesel engine mated to a proper torque converter automatic, but in the Rs 15 lakh budget, you can only get the lower-spec version of the two that misses out on features. Between these four, we would recommend the Venue since it is a newer SUV with a comfy ride and a smoother automatic gearbox.
However, if you want a car for six passengers, you need to look at manual options. The Kia Carens Clavis is available with a diesel manual option that is spacious for six/seven, and the Maruti Suzuki Ertiga with a CNG and a 5-speed manual gearbox.