A limiting nutrient is one that is either scarce or cycles very slowly. This makes ecosystem deficit of such nutrient.
Nitrogen compounds contain nitrogen in its chemical structure. Some of the nitrogenous compounds are ammonia, nitrate, nitrites, proteins, etc. Hence, the option (a) is incorrect.
Organic phosphate is the phosphate that is been absorbed by the plant roots and been converted into organic form. Such phosphate moves throughout the food web. Hence, the option (b) is incorrect.
Biogeochemical cycle refers to the movement of nutrients in the biological, geological and chemical components of the biosphere. Hence, the option (c) is incorrect.
Hence, the correct answer is option (d) limiting nutrient.