The correct option is B Mg++
Magnesium is an essential macronutrient for plants. Magnesium is a constituent of chlorophyll and is required by a number of enzymes, as cofactor. In fact for all the enzymes, which involve phosphorylation or dephosphorylation i.e., either synthesis or breakdown of ATP require magnesium as cofactor. For example all the kinase enzymes, like hexokinase, glucokinase etc.