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Water is a ____ made up of hydrogen and oxygen combined in the ratio ____ by volume or _____ by mass.
1.Compound, 3:2, 1:1 respectively.
2.Element, 1:2, 3:1 respectively.
3.Compound, 2:1, 1:8 respectively.
4.Molecule, 1:1, 1:16 respectively.

Answer
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Hint: Molecule: The substance which is formed by same type of elements i.e. same metals or same non-metal.
Compound: The substance which is formed by two or more different elements are known as compounds.

Complete step by step solution:
Let us first talk about elements, molecules and compounds.
Element: It is defined as the smallest part of any substance. They are made of the same type of atoms. They are basically of two types i.e. metals and non-metals. For example: hydrogen, carbon, oxygen, etc.
Molecule: The substance which is formed by the same type of elements i.e. same metals or same non-metal. For example: hydrogen molecule i.e. H2 (which is made of two hydrogen atom), oxygen molecule i.e. O2 (which is made of two oxygen atoms), etc.
Compound: The substance which is formed by two or more different elements are known as compounds. For example: water i.e. H2O (which is made of two hydrogen elements and one oxygen element), carbon dioxide i.e. CO2 (which is made of two oxygen and one carbon atom), etc.
Now we know that the molecular formula (which tells the composition of elements in the compound) of water is H2O. By the formula it is clear that in one molecule of water there are two hydrogen atoms and one oxygen atom. So the ratio of hydrogen and oxygen by volume is 2:1.
Now molar mass is defined as the mass of one atom of the element.
We know that the molar mass of hydrogen is 2 and molar mass of oxygen is 16. So the ratio of hydrogen and oxygen by mass is as 2:16=1:8.
Hence, water is a compound made up of hydrogen and oxygen combined in the ratio 2:1 by volume or 1:8 by mass.

So option C is correct.

Note:
Atomic mass: The mass of an atom is known as atomic mass. It is represented by the symbol A.
Atomic number: It is defined as the number of protons in an atom. It is represented by the symbol Z.