The correct answer is option 4: Magnification decreases with the increase in focal length of the eyepiece.

To understand this, we need to know how an optical microscope works. An optical microscope consists of two main components: the objective lens and the eyepiece. The objective lens is located near the specimen and focuses the light coming from the specimen onto the intermediate image. The eyepiece is located near the viewer`s eye and magnifies the intermediate image further.

In an optical microscope, the total magnification is the product of the magnification of the objective lens and the magnification of the eyepiece. So, if we want to increase the total magnification, we can either increase the magnification of the objective lens or the magnification of the eyepiece.

The focal length of the eyepiece is inversely related to its magnification. This means that a longer focal length eyepiece will have a lower magnification and a shorter focal length eyepiece will have a higher magnification.

Therefore, as mentioned in option 4, the magnification decreases with an increase in the focal length of the eyepiece.