Option- 1: "Both the eyepiece and objective of a microscope are convex lenses." - This statement is correct. In an optical microscope, the eyepiece and objective lenses are both convex lenses. The objective lens is located near the specimen being observed and forms a real, inverted, and magnified image of the specimen. The eyepiece lens is located at the top of the microscope and is used to view the image created by the objective lens.

Option- 2: "The magnification of a microscope increases with an increase in the focal length of the objective." - This statement is not mentioned in the question and cannot be determined from the given information.

Option- 3: "The magnification of a microscope depends upon the length of the microscope tube." - This statement is correct. The magnification of a microscope is determined by the ratio of the focal length of the objective lens to the focal length of the eyepiece lens. The length of the microscope tube plays a role in determining this magnification.

Option- 4: "The eyepiece of a microscope is a concave lens." - This statement is incorrect. The statement contradicts the fact that the eyepiece of a microscope is a convex lens, as mentioned