B. Weathering of rocks.
Explanation - Phosphorous cycle occurs in following steps:
1. Phosphorous from the rocks get eroded and the phosphate ion from it seeps into the water and soil.
2. Plants absorb the phosphate from their roots and include it in its nucleic acid.
3. Animals feed on plants and get phosphorous from it.
4. Decomposers breaks down the phosphorous from the animals and release it into the soil which gets reabsorbed by the plants.
5. Dissolved phosphorous that got settled at the bottom of the water body remains there until uplift, and erosion of the rock continues the cycle.