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

Correct Option (d)

The 15 °C January isotherm is recognized as the climatic boundary that divides India into tropical and subtropical zones.

• Areas south of this isotherm experience average January temperatures above 15 °C, characterizing them as tropical.
• Areas north of this isotherm experience average January temperatures below 15 °C, characterizing them as subtropical.

This temperature-based division influences various aspects, including:

• Agricultural practices, such as the cultivation of tropical crops like rice in the south versus temperate crops like wheat in the north.
• Natural vegetation types.
• General winter weather patterns across the Indian subcontinent.

Incorrect Options

The isotherms of 21 °C, 18 °C, and 12 °C do not serve as the established climatic boundary for demarcating tropical and subtropical zones in India based on January temperatures. While these isotherms may represent other temperature conditions or boundaries for different classifications, the 15 °C isotherm is specifically used for this particular climatic division.