Which one of the following combination of ion will have highest polarisation?
(1) Pb​2+, Br- (2) Pb4+, Br- (3) Fe2+, Br- (3) Fe3+, Br-

The correct option would be (2) that is Pb​4+ and Br-.

This happens because if you refer to the FAJAN's rule, according to which ,cation with high charge to radius ratio posses more polarising power. 
It means the cation with a high charge and a smaller radius is more polarising. 
So Pb2+ and Fe2are out.  Now we are left with Pb4and Fe3+. Out of these two cations, Pb4+ posses a smaller radii,due to the presence of both d and f orbital electrons,which as you know have  poor shielding effect due to which effective nuclear charge increases which results in decrease in the size of the cation more effectively as compared to presence of only d electrons in Fe3+.

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