The apparent increase in the size of a lemon kept in water in a glass tumbler is due to the phenomenon of refraction of light. Refraction occurs when light passes from one medium to another, causing the light rays to change their direction. In this case, when light travels from the air into the water, it undergoes refraction.

The refraction of light causes the light rays to bend as they enter the water, and again as they leave the water and enter the air inside the glass tumbler. This bending of light is responsible for the apparent change in the size and position of the lemon when viewed through the glass tumbler.

When light rays pass through a convex lens, which is what the curved shape of the glass tumbler acts as, they converge or come together. Due to this converging effect, the light rays coming from different points on the lemon are brought closer together, making the lemon appear larger than it actually is.

Therefore, the correct answer is option 3: refraction of light.