In which property do $d$ and $l$ isomers differ?

How many optical isomers are possible for the compound $CH_3CH(COOH)-CH(COOH)CH_2CH_3$?

Given below are two statements.
Statement $I$: The compound shown below is optically active.
[Image of compound $A$]
Statement $II$: The compound shown below is the mirror image of the above compound $A$.
[Image of compound $B$]
In the light of the above statements,choose the most appropriate answer from the options given below.

Which one of the following acids does not exhibit optical isomerism?

Dichloroethene cannot exhibit ............

The necessary and sufficient condition for the existence of enantiomers is ...........