Which of the following mineral ion is used in the activation of enzymes responsible for respiration and photosynthesis? 

Enzymes are biocatalyst which speeds up the reaction of the process. It is absorbed by plants in the form of divalent Mg²⁺. It activates the enzymes of respiration, photosynthesis, and are involved in the synthesis of DNA and RNA. Magnesium is a constituent of the ring structure of chlorophyll and helps to maintain the ribosome structure.