Which of the following plant nutrients is important for the production of chlorophyll? 

The nutrient that is important for the production of chlorophyll is magnesium.

Chlorophyll is the pigment that gives plants their green color, and it plays a crucial role in photosynthesis.

Magnesium is a key component of the chlorophyll molecule, and without it, plants cannot produce chlorophyll effectively. Iron is important for plant growth, but it is not directly involved in chlorophyll production. Zinc and calcium are also important nutrients for plants, but they are not directly involved in chlorophyll production either.