The correct answer is C. Carbon.
Carbon is the only element that can form stable bonds with itself to create long chains and rings. This ability is called catenation. Carbon atoms can bond with up to four other atoms, and they can bond with each other in a variety of ways. This flexibility allows carbon to form a wide range of compounds, from simple hydrocarbons to complex organic molecules like proteins and DNA.
Aluminium is a metal that is not very reactive. It does not form bonds with itself, and it does not form bonds with other elements very easily. Hydrogen is a gas that is very reactive. It can form bonds with many different elements, but it does not form bonds with itself. Nitrogen is a gas that is not very reactive. It does not form bonds with itself, and it does not form bonds with other elements very easily.
The ability of carbon to form bonds with itself is what makes it so unique and important. Carbon is the basis of all life on Earth, and it is also used in a wide variety of industrial processes.