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.
