The correct option is D 144
Since there are 6 letters and there should be 2 letters between R and D.
∴R and D can be at
(1) 1,4 position
(2) 2,5 position
(3) 3,6 position
Now the remaing letters can be arranged in 4! ways, also R and D can be interchanged bwtween themselves in 2! ways.
∴ Number of required arrangements will be =2!×4!+2!×4!+2!×4!=3×2!×4!=144