A similarity between optical and geometrical isomerism is that 

It is given to find out the similarity between optical and geometrical isomerism. 
Explanation

Where there is restricted rotation about a bond, geometric isomers can develop. On the left and right sides of the double bond, there must be two distinct compounds.
Optical isomers are two compounds that have non-superimposable mirror images but the identical number and types of atoms, bonds, and spatial configurations of the atoms.

Both these, optical and geometrical isomerism are included in stereoisomerism.

Hence option(C) both are included in stereoisomers is correct.