Goc

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b, because, Cl having large size than F, &rest follow electronegative order

 -I effects work for CHBr3 and CHI3.

CHBr3 is more acidic than CHI3.

But For CHF3 and CHCl3, CHCl3 is more acidic

The reason is CF3- is stabilize only by -I effect.

CCl3- stabilize both by -I effect and attraction of electron by empty vacant d orbital of Cl.

F has not d orbital so only stabilize by -I effect.

So actual order of acidic strength is

CHCl3, CHF3,CHBr3,CHI3( in decreasing order)