Answer: Option (b) is the correct answer.
Step-by-step explanation:
When two or more atoms of different elements are chemically combined together in similar composition then the product formed is known as a compound.
For example, NaCl is an ionic compound.
Mixture is the substance in which two or more different components are mixed together in different composition. Components of a mixture never combine chemically.
For example, mud in water is a mixture.
Atom is defined as a single molecule of an element. For example, Na (sodium) atom.
A colloid is defined as a mixture in which insoluble particles are microscopically dispersed and suspended throughout another substance.
For example, milk is a colloid.
Therefore, we can conclude that compounds are two or more elements that have been chemically combined.