The correct answer to this question is option 4: Magnesium. Chlorophyll, the pigment responsible for the green color in plants, contains a metal constituent, and that metal is magnesium. Magnesium is an essential nutrient for plants, and it plays a crucial role in the process of photosynthesis. It is a key component of the chlorophyll molecule, which is involved in capturing light energy from the sun and converting it into chemical energy.

Magnesium is found in the center of the chlorophyll molecule, surrounded by a complex ring structure called a porphyrin. This arrangement allows magnesium to absorb light in the red and blue parts of the electromagnetic spectrum, which are essential for photosynthesis.

While iron, potassium, and manganese are also important nutrients for plants, they are not the metal constituent of chlorophyll. Therefore, options 1, 2, and 3 are incorrect.

In summary, chlorophyll contains magnesium as its metal constituent, which is essential for the process of photosynthesis in plants.