The correct option is D 3256cm
The length of the telescope, LD=f0+ue, as the objective forms the image at its focus, as the rays are from infinity, and this image acts as the object for the eyepiece.
Now, using lens formula, and using the fact that the eyepiece forms image at near point, that is v=D(=−25cm):
1f=1v−1u.
Applying sign convention, 1ue=−1D−1fe.
Or, considering only the modulus, ue=feDfe+D.
Therefore, LD=f0+ue=f0+feDfe+D=50+5×25(5+25)=3256cm.