The elemental composition of an adult human body by mass is

The elemental composition of the adult human body by mass is dominated by four primary elements: oxygen, carbon, hydrogen, and nitrogen, which together account for approximately 96% of total body mass [1]. Oxygen is the most abundant element, comprising about 61% to 65% of body mass, primarily due to the high water content in human tissues. Carbon follows as the second most abundant element at approximately 18% to 23%, serving as the structural backbone for organic molecules like proteins, fats, and carbohydrates. Hydrogen ranks third, making up about 9.5% to 10% of the mass [1]. Nitrogen is the fourth most abundant, contributing roughly 2.6% to 3.3% of the body's mass, and is a critical component of amino acids and nucleic acids [1]. Therefore, the correct descending order by mass is Oxygen > Carbon > Hydrogen > Nitrogen.

Sources

1. [1] https://nigms.nih.gov/biobeat/2015/07/elements-that-keep-us-alive-also-give-color-to-fireworks