hey thanks to all
i used
1.25 √Receiver Ht +
1.25 √Transmitter Ht instead of
1.23 which also made a lot of difference
using 1.25 in the 1st numerical gives ans = 281nm (which is one of the option but is WRONG)
and 1.23 gives the perfect answer =
276nm
Also didnt knew the difference in
theoretical maximum range and
approximate maximum range 
but now all clear..
thanks again to all