The black cotton soil of India has been formed due to the weathering of ​

Black cotton soil, or regur, is formed mainly by the weathering of the Deccan Trap basalt, which is a fissure volcanic rock. Those lava flows came out through long cracks in the crust during the Late Cretaceous to Eocene period, spreading over a large part of the peninsula. Basalt weathers into fine smectite (montmorillonite-type) clay, which retains moisture, swells when wet and cracks when dry. It is rich in iron, lime, magnesia and potash but poor in nitrogen, phosphorus and organic matter. It covers much of Maharashtra, Gujarat, Madhya Pradesh, Karnataka, Telangana and parts of Andhra Pradesh, which is why it suits cotton.

• A (brown forest soil): This is a soil type found in hilly and forested areas such as the Himalayan slopes, not the parent material of regur.
• C (granite and schist): Weathering of these crystalline rocks gives red and yellow soils of the peninsula, not black soil.
• D (shale and limestone): Sedimentary rocks do not form the typical regur belt. Limestone can add lime to soils, but it is not the main source.

Mains link: soil classification, agro-climatic zones and crop suitability (cotton, jowar, linseed) in GS-I and GS-III.

Trap: Candidates pick the option that merely sounds like a soil name, or assume any volcanic rock fits the black colour. The correct link is specifically basalt from fissure eruptions.