Classify which of the following pairs is compound and mixture?
A) Air, Ocean water
B) Water, Glucose
C) Glucose, Salt
D) Water, Air

Hint: Compound is fixed composition and mixture is variable composition. Compound formation based on Law of constant proportions and Law of multiple proportions.

Complete step by step solution:
Mixture is defined as the product of one substance distributed randomly with another substance without undergoing any chemical reaction.
Mixtures are of two types’ homogenous mixture and heterogeneous mixtures
Homogenous mixtures are mixtures which have the same proportion of components throughout the given sample.
Heterogenous mixtures are mixtures which are having different proportions of components throughout the given sample.
Compound is a pure substance which is formed by the combination of two atoms in the whole number ratio.
First option is Air, ocean water. Air is containing different gases such as Oxygen, Nitrogen, Hydrogen, dust particles, Carbon dioxide and water. So Air is considered a mixture.
Ocean water has dissolved salts in it and dissolved gases and minerals, so Ocean water is also considered as Mixture.
Second option Water and Glucose. Water chemical formula is ${H_2}O$and it is a fixed composition , the ratio of Hydrogen is 1:8. So water is called a compound. Glucose chemical formula is ${C_6}{H_{12}}{O_6}$.
Glucose is also formed by 6 Carbon atoms, 12 Hydrogen atoms and Six Oxygen atoms.
So Glucose is also considered as a compound.
Third option is Glucose, Salt. Glucose is a compound which is already discussed in the second option and coming to NaCl, Its chemical name is Sodium chloride commonly called Table salt. Sodium chloride is formed by the combination of one Sodium ion and one chlorine ion. So it is also Compound which is a fixed composition.
Fourth option is Water, Air So the water formula is ${H_2}O$, this is compound and Air is mixture and it consists of Oxygen , Nitrogen , Carbon dioxide , Inert gases and dust.

Hence, the answer is (D) water, air.

Note: Mixing of two compounds can also become compound. So Calcium oxide and Carbon dioxide combine and form Calcium carbonate. So calcium carbonate is also compound.