The correct option is A D/F
Given: in a simple microscope, the final image is formed at the infinity.
We know that, least distance for distinct vision in the simple microscope is D=25 cm.
Again we know, when the final image in a simple microscope is formed at infinity, the object must be situated at the focus F.
∴ When the image is at infinity, the magnifying power of the microscope is
M=DF
So, the correct option is (A).