The correct answer is option 4, Calcium.
Chlorophyll is a green pigment found in plants that is essential for photosynthesis. It is made up of various components, including carbon, hydrogen, and magnesium. Carbon is a fundamental element found in all organic compounds, including chlorophyll. Hydrogen is also a common element found in many organic compounds, including chlorophyll. Magnesium is a crucial component of the chlorophyll molecule and plays a vital role in capturing light energy for photosynthesis. However, calcium is not a part of the chlorophyll molecule. While calcium is important for plant growth and development, it is not directly involved in the process of photosynthesis or the structure of chlorophyll.