Two or more compounds having the same molecular formula but different arrangement of atoms are called isomers and the phenomenon is called isomerism.

Because of different arrangement of atoms, isomers differ in one or more physical or chemical properties. Isomers can be broadly classified into two major categories:

(A) Structural isomers (B) Stereoisomers.

These are further sub-divided as shown below:

Exchange of coordination group by a water molecule in complex molecule results in

Hydrate isomerism

The correct answer is (3) hydrate isomerism.

Hydrate isomerism is a type of isomerism that occurs when a complex compound has different numbers of water molecules coordinated with the metal ion. The water molecules are not part of the coordination sphere, but they can be exchanged with other water molecules or with other ligands. This results in isomers that have the same chemical formula but different physical properties.

The other options are incorrect. Option (1), ionization isomerism, occurs when a complex compound has different counter ions. Option (2), ligand isomerism, occurs when a complex compound has different ligands in the coordination sphere. Option (4), coordination isomerism, occurs when a complex compound has different metal ions in the coordination sphere.