The correct answer is option 1 - Magnesium.
The green pigment in leaves, known as chlorophyll, is responsible for absorbing sunlight during photosynthesis. Chlorophyll is made up of a porphyrin ring structure, which contains a central atom of magnesium. This magnesium atom plays a crucial role in capturing and transferring energy from sunlight.
Option 2 - Iron, is not the correct answer. While iron is an essential micronutrient for plants, it is not a part of the chlorophyll molecule.
Option 3 - Calcium, is also not the correct answer. Calcium is not involved in the green pigment of leaves. It does have important roles in plant growth and development, but it is not directly related to chlorophyll.
Option 4 - Copper, is not the correct answer either. Copper is not involved in the green pigment of leaves. Copper is required by plants in very small amounts as it serves as a cofactor for certain enzymes, but it is not part of chlorophyll.
Therefore, the only correct answer is option 1 - Magnesium.