The correct answer is option 1: Magnesium. Magnesium is a key element present in the green pigment of a leaf called chlorophyll. Chlorophyll is responsible for capturing sunlight and converting it into chemical energy through the process of photosynthesis. Magnesium is an essential component of chlorophyll`s structure, playing a crucial role in its ability to absorb light energy.

Option 2: Phosphorus is not the correct answer because phosphorus is not a component of chlorophyll. Phosphorus is primarily involved in energy transfer and storage in the form of ATP (adenosine triphosphate) and is not directly related to the green pigment of a leaf.

Option 3: Iron is also not the correct answer as iron is not a part of chlorophyll`s structure. Iron is more commonly associated with other processes in plants, such as electron transport and enzyme activities.

Option 4: Calcium is not the correct answer either, as calcium is not a component of chlorophyll in the green pigment of a leaf. Calcium has important functions in plants, but it is not directly involved in photosynthesis or the structure of chlorophyll.

In summary, the correct answer is option 1: Magnesium is the element present in the green pigment of