Concept:A pure compound always contains the same elements combined in a fixed ratio by mass.
Explanation:This definition describes the law of definite proportion, also called the law of constant composition.
It means that any sample of a particular compound, no matter its source or method of preparation, has the same elements in the same proportion by mass.
For example, pure water, whether from rain or a laboratory, always contains hydrogen and oxygen in a fixed mass ratio.
The other options are different laws: conservation of mass deals with total mass in reactions, multiple proportions involve elements forming more than one compound, and reciprocal proportions relate combining ratios between elements.
Therefore, the statement matches the law of definite proportion.
Answer:B. law of definite proportion