Which element(s) is/are mobile with plant system?

All the given elements Chlorine, Molybdenum and Potassium are mobile with the plant system.
The upward movement of calcium takes place in the transportation stream through the xylem. The calcium ions do not move by mass flow but by a series of exchange reactions along negatively charged sites on the wall of the xylem vessels.
They move from one place to another when there is any deficiency detected in plants. Plant moves mobile nutrients from older issues to newer tissues.