"Pure water is always 11.2% H and 88.8% O by mass" statement best supports the law of definite proportions.
Option: C
Step-by-step explanation:
The law of Proust is basically about chemical compound which are comprises of its constituent elements in a fixed ratio but on origin and preparing process it does not rely. This law is also named as law of a definite proportion or composition or constant composition.
The atom of oxygen has an atomic mass of 16 while an atom of hydrogen has an atomic mass of 1. This implies that water consists of 11% hydrogen, and 89% oxygen. To produce water, the very same proportion of hydrogen and oxygen should always be mixed.