Irrespective of the source, a pure sample of water always yields 88.89% mass of oxygen and 11.11% mass of hydrogen. This explained by the law of :
a.) Conservation of mass
b.) Constant composition
c.) Multiple proportion
d.) Constant volume
e.) Gay - lussac’s law

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Hint: In the question, mass proportion is told constant. So, the correct answer will be related to constancy in proportion. The option that will show the constancy in the proportion of elements will be the correct answer.

Complete step by step answer:
We have studied the laws of chemistry. We know that the law of constant composition suits here.
The law of constant composition says that in any particular compound, all samples will be made up of the same elements in the same composition or ratio irrespective of the source. Thus, any water molecule will always be made up of two hydrogen atoms and one oxygen atom in a 2:1 ratio. This ratio will be the same for all water molecules from whichever source does the water comes.
In the question also, it is said that the proportion of hydrogen and oxygen is also the same from whichever source sample is taken. So, the law of constant composition explains this.
Thus, option b.) can be the answer.

Now for the other options; the option a.) is the conservation of mass according to which the sum of masses of all reactants is equal to the sum of masses of all the products.
This option does not suit and thus, option a.) is incorrect.
Now, for option c.) which is a law of multiple proportions. According to this law, in case two elements are forming more than two compounds then the ratio of the masses of the second element which will combine with a fixed mass of the first element will always be in whole numbers.
So, even option c.) is not fitting well here.

Now, the option d.) which is constant volume. There is no law for this. The process in which volume is constant is called isovolumetric process.

Now, the last option e.) which is Gay - lussac’s law. It says that The pressure of a gas is directly proportional to the absolute temperature of the gas when volume is constant. There is no such thing in question.

So, the correct answer is option (b)

Note: It does not even mean that hydrogen and oxygen will always combine in a 2:1 ratio forming ${H_2}O$. It may also combine in multiple proportions forming other molecules. For example, hydrogen and oxygen may combine in different proportions to form ${H_2}{O_2}$ rather than ${H_2}O$. In ${H_2}{O_2}$, Hydrogen and Oxygen atoms are in 1:2 ratio.