Concept:The law of definite proportions describes the fixed composition of a pure chemical compound.
Explanation:This law was proposed by Proust.
It states that every pure compound always contains the same elements.
These elements are always present in a fixed proportion by mass.
Thus, any pure sample of a given compound has exactly the same elemental composition.
Option A correctly states this meaning.
Option B is incomplete and unclear.
Option C mentions only purity, not proportion by mass.
Option D refers to the law of conservation of mass.
Therefore, the best choice is A.
Answer:A. pure samples of the same compound contain the same elements combined in the same proportion by mass.