When viewing a cork through his homemade microscope, robert hook discovered small compartments that he called
boxes
cells
prokaryotes
eukaryotes

For this problem, the most probable and the most likely answer for this would be the second option which is cells.

When viewing, not just corks, but other things under a microscope, may it be a home-made one or of something in value or a technologically advanced microscope, small compartments would always be visible under these lenses and they are cells. Some might look like boxes but they are cells. These cells might come in many sizes, but they will always be cells. Cells are everywhere.