Concept:Magnesium is a central component of the chlorophyll molecule.Explanation:Chlorophyll is the green pigment responsible for photosynthesis.Each chlorophyll molecule contains a magnesium ion at its centre.Without magnesium, chlorophyll cannot be produced correctly.Plants lacking magnesium show yellowed leaves, known as chlorosis.Thus, magnesium is essential for chlorophyll formation.Answer:D. magnesium