Magnesium is needed by plants for

Magnesium is needed by plants for the formation of chlorophyll, which is the pigment responsible for the green color in leaves and is essential for photosynthesis. Photosynthesis is the process by which plants produce their own food using sunlight, carbon dioxide, and water. Chlorophyll is a critical component of this process as it captures the energy from sunlight and converts it into chemical energy that the plant can use to fuel its growth and metabolism. Magnesium is a key component of the chlorophyll molecule, and without it, plants are unable to produce chlorophyll. This can lead to a condition called chlorosis, where the leaves of the plant turn yellow or white due to a lack of chlorophyll. Therefore, the correct option to the question is "formation of chlorophyll" as magnesium is required for the synthesis of this essential pigment. The other options listed (protein synthesis, formation of cell membrane, and activation of enzymes) are also important functions of magnesium in plants, but they are not directly related to the formation of chlorophyll.