The correct option is D Calcium
Soil is the reservoir of nutrients for the plants, however the availability of nutrients in the soil fluctuates. Thus in higher plants the nutrients are mobilised from older, senescing leaves to younger leaves. This mechanism is called remobilization. It aims at minimizing the mineral nutrient deficiency. Remobilization can also be seasonal where nutrients like nitrogen and phosphorus are remobilized from the leaves before leaf shedding.
Mineral ions like phosphorus, sulphur, nitrogen, potassium are frequently remobilised from older parts of plants like old dying leaves, to younger parts like young leaves.
Certain nutrients that are a structural component of the plants cannot be remobilised. Calcium is associated with the cell plate in plants and is a structural component in the plant cells. Thus remobilization of calcium does not take place.