In case of which one of the following biogeochemical cycles, the weathering of rocks is the main source of release of nutrient to enter the cycle? ​

Phosphorus has no significant atmospheric reservoir. Its main reservoir is sedimentary rock (apatite and phosphate rock), so the cycle effectively starts when weathering releases phosphate into soil and water. That makes C correct.

• Carbon cycle (A): The atmosphere (CO2) and oceans are the major reservoirs. Silicate weathering does draw down CO2 over geological time, but it is not the main entry route into the active cycle; photosynthesis and respiration dominate.
• Nitrogen cycle (B): The atmosphere holds about 78% N2. The main entry routes are biological nitrogen fixation, lightning and industrial fixation. Rock weathering contributes very little.
• Sulphur cycle (D): Sulphur enters mainly through volcanic emissions, decomposition and fossil fuel combustion, with a significant atmospheric component (SO2, DMS). Weathering contributes but is not the main source.

Core concept: gaseous versus sedimentary cycles. Carbon, nitrogen and sulphur are gaseous cycles with atmospheric reservoirs; phosphorus is a sedimentary cycle. For Mains GS-III (environment, fertilisers, eutrophication), this explains why phosphate fertiliser runoff causes eutrophication and why phosphate rock is a finite resource.

Trap: Students often pick carbon because weathering is linked to the carbon-silicate cycle. The question asks about the main source of release into the cycle, and for carbon that is the atmosphere and biosphere.