The correct answer is: Carbon.
Carbon is the most abundant element by number in the living system. It is the basic building block of organic compounds, which are the molecules that make up living things. Carbon atoms can bond to each other in a variety of ways, which allows them to form a wide range of different compounds. This versatility is one of the reasons why carbon is so essential for life.
Nitrogen is the second most abundant element by number in the living system. It is a component of proteins, nucleic acids, and other important molecules. Nitrogen is also essential for the process of photosynthesis, which is how plants use sunlight to create food.
Oxygen is the third most abundant element by number in the living system. It is a component of water, which is essential for life. Oxygen is also used in cellular respiration, which is the process by which cells produce energy.
Hydrogen is the fourth most abundant element by number in the living system. It is a component of water and organic compounds. Hydrogen is also used in cellular respiration.
In conclusion, the most abundant element by number in the living system is carbon. Carbon is the basic building block of organic compounds, which are the molecules that make up living things. Carbon atoms can bond to each other in a variety of ways, which allows them to form a wide range of different compounds. This versatility is one of the reasons why carbon is so essential for life.