Please help me with this question please

Dear student
Lower the pKa value more is the acidic strength 
So here we have to find the structure with lowest pKa which means we have to find the structure with highest acidic strength
in 1) benzyl ion is formed after removal of H+ ion which is stable upto an extent 
2) here the carbocation formed is more stable than 1
In the structure 3rd and 4th if we compare the carbocation formed after removal of H+ ion the only difference is that in the 3rd option the structure will get distorted as all 3 benzene cannot remain in the same plane  while in 4th one there is no such restriction 
hence 4th structure is must stable one

Regards   
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