The four essential elements for life are a. carbon, hydrogen, oxygen and nitrogen b. carbon, hydrogen, oxygen and sulfur c. carbon, nitrogen, oxygen and sulfur d. iron, nitrogen, oxygen and magnesium

The correct answer is "A. carbon, hydrogen, oxygen and nitrogen".

Scientists believe that out of the 116 known elements, only 25 are used by mammals (and therefore by humans). Out of the 25 elements that are believed by scientists to support human life, only four of these elements - carbon, oxygen, hydrogen, and nitrogen - make up about 96% of the human body and are found in the basic structure of all biochemical molecules within the human body. Because of this, carbon, hydrogen, oxygen, and nitrogen are considered "the four essential elements" supporting human life. However, it is also important to keep in mind that although many of the other 25 elements present in the human body are required in only small amounts, they play an important role in keeping the body functioning correctly, as well.