I'd say it is phosphorus or phosphate. The easiest way to identify the answer is trough these frases:
Plants absorb this nutrient from the soil and turn it into organic compounds through assimilation.
Plants take up the usable nutrient through the soil and assimilate it into proteins, making it part of the plant.
During assimilation, plants provide phofphorous (and other nutrients) to the cells and tissues - plants absorb this nutrient from the soil and turn it into organic compounds through assimilation.
Plants also used phosphorus in processes such as transpiration and photosynthesis wich contributes to the fact that plants take up the usable nutrient through the soil and assimilate it into proteins, making it part of the plant.
Hope it helped,
Happy homework/ study/ exam!