See there are 3 steps to this :
a) Before entering into CLA circuit , n AND gates and n XOR gates for generator and propogator functions required for ai and bi operand values..
b) In the CLA circuit , we have 2 level implementation of AND gates followed by OR gates :
So in that no of AND gates required = n(n+1) / 2
And no of OR gates = n
Therefore for 4 bit carrylookahead adder ,
No of AND gates in total required = 4 + (4*5/2)
= 14
No of OR gates = 4
Therefore 2X = 2*14 = 28
and 2Y = 2*4 = 8
So 2X + 2Y = 28 + 8
= 36
Hence 36 is the correct answer..