Which of the following is/are  a structural isomerism
A) Ionisation isomerism 
B) Ligand isomerism
C) Hydrate isomerism
D) Geometrical isomerism
 

Ionization isomerism has the same composition but produces distinct ions in a solution. In ligand isomerism, the ligand involved is isomeric in nature.

Hydrate isomerism arises when different no. of solvate molecules are connected to the metal ion. So, no structural difference is found. 
Geometrical isomerism has the same chemical formula but different arrangements of the atoms in space. and is not a structural isomerism