The correct answer is C. Nitrogen.
Nitrogen is an essential element for plant growth. It is a component of proteins, nucleic acids, and chlorophyll. Plants need nitrogen to produce new cells and tissues.
Insectivorous plants grow in soils that are deficient in nitrogen. This is because these plants have adapted to obtain nitrogen from insects. They trap insects with their leaves and then digest them using enzymes.
The other options are incorrect because they are not essential elements for plant growth. Magnesium is a component of chlorophyll, but plants can obtain enough magnesium from the soil. Calcium is a component of cell walls, but plants can obtain enough calcium from the soil. Water is essential for plant growth, but plants can obtain enough water from the soil.
In conclusion, the soil in which insectivorous plants grow in deficient in nitrogen. This is because these plants have adapted to obtain nitrogen from insects.