Which of the following elements is responsible for maintaining turgor in cells?

Potassium is needed to maintain turgor of plant cells.
(1) Magnesium is needed for the activation of many enzymes of respiration, nucleic acid synthesis and maintenance of
ribosome structure. It is a structural component of chlorophyll. 
(2) Sodium is a non-essential and benefi coal element which mainly plays a role in C4 plants. 
(4) Calcium is needed for cell division, cell elongation and pollen germination. It is a structural component of middle lamellum.