To determine which molecule is most common in the human body, let's examine each of the given options:
A. [tex]\( H_2 \)[/tex]: This represents diatomic hydrogen, which is not prevalent in significant quantities in the human body.
B. [tex]\( CO_2 \)[/tex]: This stands for carbon dioxide, a molecule produced as a byproduct of cellular respiration. While it is present in the body, it is expelled through the respiratory system and does not constitute a large proportion of the body's molecular composition.
C. [tex]\( H_2O \)[/tex]: This represents water. Water is a major component of the human body, making up about 60% of total body weight in an average adult. It is vital for numerous bodily functions, including temperature regulation, transportation of nutrients, and chemical reactions within cells.
D. [tex]\( N_2 \)[/tex]: This refers to diatomic nitrogen, which is the major component of the Earth's atmosphere. Although nitrogen is essential for the formation of amino acids and proteins, as a molecule [tex]\( N_2 \)[/tex], it is not abundantly found in the human body in comparison to water.
Given these considerations, the most common molecule in the human body is water ([tex]\( H_2O \)[/tex]). Therefore, the correct answer is:
C. [tex]\( H_2O \)[/tex]
