The correct answer is option 3 - 100 micron.

To understand why this is the smallest size of a cell that can be seen with the naked eye, we need to have some background knowledge about the size of cells and the capability of the naked eye to perceive objects.

Cells are the basic building blocks of life and can vary in size. The average size of a human cell is around 10 to 30 microns, which is smaller than what can be seen with the naked eye.

The naked eye, without the aid of any magnifying devices, can typically perceive objects that are larger than 100 microns. This is because the naked eye has limited resolution and cannot distinguish details smaller than a certain size.

Therefore, anything smaller than 100 microns, such as individual cells or cellular structures, would not be visible to the naked eye.

It`s important to note that there are exceptions to this general rule, such as the visibility of some larger types of cells like eggs or parasites. However, in the context of the answer choices given, option 3 - 100 micron is the most accurate and widely accepted answer.