The correct answer is oxygen. Oxygen makes up about 65% of the human body by mass, followed by carbon (18%), hydrogen (10%), nitrogen (3%), calcium (1.5%), phosphorus (1%), potassium (0.4%), sulfur (0.3%), sodium (0.2%), chlorine (0.15%), magnesium (0.1%), iron (0.08%), zinc (0.07%), iodine (0.05%), selenium (0.003%), fluorine (0.001%), and chromium (0.0002%).
Oxygen is essential for life because it is the gas that cells use in respiration to produce energy. Carbon is the main component of organic molecules, such as proteins, carbohydrates, and lipids. Hydrogen is also found in organic molecules, and it is essential for the formation of water. Nitrogen is a component of proteins and nucleic acids. Calcium is a mineral that is found in bones and teeth. Phosphorus is a mineral that is found in bones and teeth, and it is also involved in energy production. Potassium is a mineral that is involved in muscle contraction and nerve function. Sulfur is a component of proteins and some vitamins. Sodium is a mineral that is involved in fluid balance. Chlorine is a mineral that is involved in fluid balance. Magnesium is a mineral that is involved in muscle contraction and nerve function. Iron is a mineral that is involved in the formation of hemoglobin, which carries oxygen in the blood. Zinc is a mineral that is involved in the formation of proteins and enzymes. Iodine is a mineral that is involved in the formation of thyroid hormones. Selenium is a mineral that is involved in the formation of enzymes. Fluorine is a mineral that is involved in the formation of tooth enamel. Chromium is a mineral that is involved in the metabolism of glucose.
The remaining elements are found in trace amounts in the human body.