The correct option is Kanker.
Explanation
In tropical and subtropical climates with intermediate rainfall, chemical weathering and the movement of groundwater play a significant role in soil composition. In the Indo-Gangetic plains, specific soil horizons accumulate minerals due to the leaching and precipitation of lime.
- Hardpans is incorrect: Hardpans refer to general dense layers of soil, usually found below the topsoil, which are impervious to water. While they can be cemented by calcium carbonate, silica, or iron, "hardpan" is a generic geological term rather than the specific local term for the nodules described.
- Kanker is correct: Kanker (or Kankar) is the specific local term used in the Indian subcontinent to describe nodules of impure calcium carbonate found in Bhangar (old alluvium) soils. These nodules form due to the precipitation of calcium carbonate within the soil profile.
- Peat is incorrect: Peat is an accumulation of partially decayed vegetation or organic matter. It forms in wetland conditions and is organic in nature, distinct from inorganic mineral concretions.
- Humus is incorrect: Humus is the dark, organic component of soil formed by the decomposition of plant and animal matter. It is crucial for soil fertility but is chemically different from calcium carbonate nodules.
Key Takeaway: Kanker represents the calcareous (calcium carbonate) concretions found specifically in the Bhangar (old alluvium) soils of the Indo-Gangetic plains.