The compound essential for the process of photosynthesis has this element

The correct option is Mg.

Photosynthesis is the biological process by which green plants, algae, and some bacteria convert light energy into chemical energy. The key molecule responsible for capturing this light energy is the pigment chlorophyll.

At the center of the chemical structure of every chlorophyll molecule is a single magnesium ion:

$${\mathrm{Mg}}^{2+}$$

This central magnesium ion is bound within a porphyrin ring structure. It helps stabilize the molecule and plays a crucial role in transferring the absorbed light energy to drive the chemical reactions of photosynthesis.

Without magnesium, plants cannot form chlorophyll, which prevents photosynthesis from taking place. Therefore, Magnesium (Mg) is the element essential for this process.