Sayantan Sir....

that's an interesting one...
whenever u square a no. that ends with either 1, 5 or 6....u will always get the same digit at the unit's place, i.e. 1,5 and 6 respectively...
now, why i was telling you about this is because after squaring the given no. the unit digit so obtained is the same..so, possibly either we are squaring 11, 55 or 66.
square of 11 will give me 121 which contains only three digits...so not this one
square of 55 will give me 3025...it has four digits but here "B" , i.e. 2 is even...so it does not satisfies the the given condition..
so, finally we are left with 66..
square of 66 is 4356....here "B", i.e. 5 is an odd no. which satisfies the given condition...and all the digits are distinct...so that's the no..
now, obviously the value of "D" will be-
DCBA=4356
i.e.D=4...
.... :) ...