The January isotherm taken as a basis for dividing India into tropical and subtropical zones is:

Answer: (b) 18°C

The 18 °C January isotherm runs roughly along the Tropic of Cancer and divides tropical India (south) from subtropical India (north).

The idea

An isotherm joins places with the same temperature for a stated period. A January isotherm describes winter temperature, so it need not follow a fixed latitude. The commonly used thermal division between tropical and subtropical India is the 18 °C January isotherm.

Also know

A latitude line is fixed, while an isotherm bends with altitude, land–sea contrast and circulation. Isobars join equal pressure; isohyets join equal rainfall. Köppen classification uses temperature and rainfall together, so it is not the same as this single-isotherm division.