The correct answer is option 3: Magnesium.
Chlorophyll is a pigment found in green plants that is responsible for the absorption and conversion of light energy into chemical energy through the process of photosynthesis. Chlorophyll molecules contain a magnesium ion at the center, which is essential for their structure and function.
Option 1: Calcium is an important mineral for plants and is involved in various physiological processes, but it is not directly involved in the formation of chlorophyll.
Option 2: Iron is also an essential mineral for plants, but it is not directly involved in the formation of chlorophyll.
Option 4: Potassium is another important nutrient for plants, but it is not directly involved in the formation of chlorophyll.
Therefore, the correct element that is essential for the formation of chlorophyll in green plants is magnesium.