Final answer:
The focusing knob is responsible for altering the relative distance between the specimen and the objective lens to focus the image on the retina. The correct option is E).
Step-by-step explanation:
The part of a microscope responsible for altering the relative distance between the specimen and the objective lens, thus focusing the final image precisely on the plane of the retina, is the focusing knob.
The focusing ability of a microscope is provided by the movement of both the objective lens and the eyepiece (ocular lens). To obtain the clearest image, one must adjust the focusing knobs to change the distance between the objective lens and the specimen.
As magnification increases, lenses may require adjustments to the illumination to make up for a darkening image due to smaller lens apertures at higher magnifications. These illuminations can be adjusted through light intensity, condenser lens position, and the iris diaphragm to achieve the perfect image. Option E) is the correct one.