Magnesium is present in :
A. Ascorbic acid
B. Vitamin ${{\text{B}}_{12}}$
C. Chlorophyll
D. Haemoglobin

Hint: Magnesium ( ${\text{Mg}}$ ) an element in periodic table, having atomic number of $12$ . It is an abundant mineral element in the body, and is naturally present in many foods. Magnesium is required for energy production, glycolysis and also for phosphorylation. Magnesium is widely distributed in many plants and animal foods.

Complete step by step answer:
 As we know ${\text{Mg}}$ is an important element for our body functioning and helps to maintain good health.
Here we have given ascorbic acid, Vitamin ${{\text{B}}_{12}}$, chlorophyll and haemoglobin and we have to check in which compound the element ${\text{Mg}}$ is present. So that we have learned about the structure and properties of these compounds.
Firstly we learn about ascorbic acid, ascorbic acid is mainly used to treat vitamin ${\text{C}}$ deficiency in people. Vitamin ${\text{C}}$ plays an important role in our body. It is much needed to maintain good health. It also protects our body cells from damage , and is known as antioxidants. Ascorbic acid has a molecular formula of ${{\text{C}}_6}{{\text{H}}_8}{{\text{O}}_6}$. And its structure is given by

Next is vitamin ${{\text{B}}_{12}}$ , it is a nutrient which helps to keep the body's nerves and blood cells healthy and also helps to make DNA. Vitamin ${{\text{B}}_{12}}$ found naturally in a variety of animal foods and in some fortified foods. Plant foods do not contain vitamin ${{\text{B}}_{12}}$ . Vitamin ${{\text{B}}_{12}}$ is the only essential biomolecule in which we can see a stable metal carbon bond. And the structure is based on a corrin ring with various side groups attached. The central metal ion is cobalt. The structure is given by

Next we learn about chlorophyll, where chlorophyll is a pigment which gives green colour to plants. It absorbs the sunlight and uses the energy for photosynthesis which is the basis for sustaining the life process of all plants. Since animals and humans depend on green plants for food , it can also be said photosynthesis is a source of our life also. The structure of chlorophyll is given by

Chlorophyll contains a ${\text{Mg}}$ion which defines the structure of chlorophyll.
Next we will learn about haemoglobin, it is a protein molecule found in red blood cells that carries oxygen. And its structure is given by


 So from above given structures it is clear that the compound in which ${\text{Mg}}$ present is chlorophyll.

So, the correct answer is Option C.

Note: potassium and magnesium are mineral nutrients which are required in large quantities by the plants, as both elements are the major contribution to the photosynthesis process. Without magnesium chlorophyll cannot capture the energy from sunlight which is the main requirement for photosynthesis.