Please answer 51. What is the stability order of anti aromatic compounds?
Q.51. The compound which is antiaromatic and least stable is
(1)

(2)

(3)

(4)

Anti aromatic structures have 4n  pi electrons. Where n can be any whole number. Basically if the number of pi electrons are multiples of 4, it may be an anti-aromatic. 
Aromatic structures have 4n+2 pi electrons.
Non-aromatic does not have any of the above. 

Now, the third option is ruled out because Benzene is aromatic