The correct answer is option 2: Magnesium. Chlorophyll is a pigment found in plants that is responsible for absorbing light energy for photosynthesis. It is what gives plants their green color. Chlorophyll contains a magnesium atom at the center of its structure, which is crucial for its function in capturing light energy.

Option 1, Calcium, is not present in chlorophyll and does not contribute to the green color of plant leaves. Calcium is an essential mineral for plant growth and development, but it does not play a role in the coloration of chlorophyll.

Option 3, Iron, is also not present in chlorophyll. Iron is involved in other processes in plants, such as the synthesis of heme in the production of chlorophyll, but it is not directly responsible for the green color of chlorophyll.

Option 4, Manganese, is also not present in chlorophyll. Manganese is required by plants as a cofactor for several enzymes involved in photosynthesis, but it does not contribute to the green color of chlorophyll.

Therefore, the correct answer is option 2: Magnesium, which is the element present in chlorophyll that gives a green color to plant leaves.