Which one of the following elements is not remobilized by the plants? 

Calcium is not typically remobilized by plants. While calcium is an important nutrient for plants and is taken up by the roots, it tends to be more immobile within the plant compared to other nutrients like potassium, phosphorus, and sulfur. 

Once taken up by the roots, calcium is primarily transported through the xylem (water-conducting tissue) to the shoots, where it provides structural support to cell walls. 

As a result, when calcium is deficient in the soil, it can lead to symptoms like stunted growth and leaf deformation, but these symptoms do not typically involve the translocation of calcium from older leaves to new growth. 

In contrast, nutrients like potassium, phosphorus, and sulfur are more mobile within the plant and can be remobilized from older leaves to new growth during periods of deficiency.