Explanation:
the coefficient of hydrogen is 3
It's because of Avagadro's number.
- It is known as Avagadro's constant too.
- It states that at constant temperature one mole of any substance contain same number of atoms i.e n no of atoms
where

The answer that would best complete the given statement above is the term ATOM. <span>The smallest unit which maintains the physical properties of a compound is an ATOM. But for a compound, it would be molecule. Hope this answers your question.</span>