The January isotherm taken as a basis for dividing India into tropical and sub-tropical zones is

The correct answer is 18 degree Celsius. In climatology, the 18 degree Celsius mean January isotherm is widely used as a benchmark to distinguish between tropical and subtropical climates in India. This line roughly runs parallel to the Tropic of Cancer. Regions to the south of this isotherm experience tropical conditions where temperatures generally remain warm even in winter, while regions to the north fall into the subtropical zone, where winters are significantly cooler. This classification helps in understanding the distribution of vegetation and agricultural patterns across the country.