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?

(a) Carbon cycle: Main reservoir is atmospheric CO2 and oceans; release is mainly through respiration, decomposition, combustion and ocean-atmosphere exchange, not chiefly rock weathering. Verdict: Incorrect.

(b) Nitrogen cycle: Nitrogen enters the cycle mainly from atmospheric N2 through fixation by bacteria, lightning and industrial processes. Verdict: Incorrect.

(c) Phosphorus cycle: Phosphorus has no major gaseous phase; its principal source is weathering of phosphate rocks, which releases phosphate into soil and water. Verdict: Correct.

(d) Sulphur cycle: Sulphur enters from volcanic emissions, decomposition, sea spray and also some rock weathering, but weathering is not the main defining source. Verdict: Incorrect.