Answer: Option (a) is the correct answer.
Step-by-step explanation:
An atom is defined as the smallest unit of each and everything that is surrounding us.
This means that each state of matter is made up of atoms.
For example, a chair is made up of wood and wood is made up of large number of carbon atoms.
According to Dalton's theory, matter consists of indivisible particles that is called atoms and atoms of a given element are all identical in nature which can neither be created nor destroyed.
Thus, we can conclude that the word "atom" means the smallest imaginable thing.