Which of the following elements is considered a macro element?

Calcium is considered a macro element because it is one of the essential nutrients required in relatively large quantities for plant growth and development. Macro elements play crucial roles in various physiological functions within plants, including structural support, enzyme activation, and regulation of various metabolic processes. Calcium, for instance, is vital for cell wall structure and stability, as well as for signaling processes within plant cells.

In contrast, hydrogen, carbon, and chlorine serve important functions in plants too, but they fall into different categories. Hydrogen and carbon are classified as primary elements necessary for basic organic structure—they are integral parts of carbohydrates, proteins, and nucleic acids, but they do not fit the macro element definition because they are required in much larger quantities for overall plant development. Chlorine, on the other hand, is a micronutrient, needed in smaller amounts, and primarily involved in photosynthesis and osmosis. Therefore, calcium stands out as the correct answer among the given options when identifying a macro element.