The correct answer is option 2: Chlorophyll.

Chlorophyll is a pigment found in plants and algae that is essential for photosynthesis. It captures light energy and converts it into chemical energy, which is used to produce glucose and oxygen from carbon dioxide and water. Chlorophyll plays a crucial role in the process of photosynthesis by absorbing light and transferring energy to other molecules.

Magnesium is a critical component of chlorophyll and is essential for its structure and function. It is located at the center of the chlorophyll molecule and is responsible for capturing light energy. Therefore, magnesium is present in chlorophyll.

On the other hand, the other options mentioned in the question do not contain magnesium. Ascorbic acid (option 1) is a form of vitamin C and does not contain magnesium. Hemoglobin (option 3) is the protein responsible for carrying oxygen in red blood cells and does not contain magnesium. Vitamin B12 (option 4) is another vitamin that does not contain magnesium.

In conclusion, magnesium is present in chlorophyll, making option 2 the correct answer.