Elimination of HCl from isomeric chlorocyclohexadienes
That's a fair question,

Assuming that you use a strong base for dehydrohalogenation you will be deprotonating the most acidic $\ce{H}$ resulting in formation of a carbanion which has the highest stability in compound number 2 

You can see that the carbanion formed in compound numer 1 will have resonance with only one of the double bond and hence will be less stable then the carbanion in the compound 2 which has resonance from both the double bond.

Mechanism can be as follow :-



Also;