The correct answer is option 1: Oxygen. Oxygen is the most abundant element in the human body. It makes up approximately 65% of the human body by mass. Oxygen is essential for various biological processes, including respiration, energy production, and metabolism. It is a key component of water and is also found in many organic molecules, such as carbohydrates, proteins, and fats. Oxygen is obtained through breathing and is transported throughout the body by the circulatory system. While carbon, iron, and nitrogen are also present in the body, they are not as abundant as oxygen. Carbon is the second most abundant element in the body, followed by hydrogen, nitrogen, calcium, and phosphorus.