Which one of the following element is considered to be an immobile element in the plant

Answer: Option A

Calcium is the most immobile element in the plant out of the given options.
This is because the upward movement of calcium takes place in the transpiration stream through the xylem vessels. The calcium ions do not move by mass flow but by a series of exchange reactions along negatively charged sites on the walls of xylem tissues, thus it keeps on moving and immobile.