Final answer:
The diameter of the image of the orange in the mirror will be the same as the actual orange, which is 4 cm, because a plane mirror produces an image of equal size to the object.
Step-by-step explanation:
The diameter of the image of the orange in the mirror will be 4 cm, the same as the actual orange. This is because a plane mirror creates an image that is laterally inverted and of the same size as the object. Since the orange's diameter is given as 4 cm, its image, which will appear as if it is behind the mirror at the same distance from the mirror as the orange is in front, will also be 4 cm in diameter.