Mendeleev left gaps in his original periodic table due to which of the following reasons?

Yes, e) is the answer. He originally started lining the elements up by atomic mass, since at that time they did not know about electrons, protons and neutrons, and even the idea of the existence of atoms was controversial. Also remember that the Noble Gases had not been discovered, so they weren't there. He noticed that there were gases "Periodically" throughout the list he made, so he changed the list into a table with similar materials being placed below and above each other. In some cases, this arrangement didn't quite work and he had to slide the row over by one spot to keep properties the same. This created a hole that he said had to be filled.