Question

why couldnt mendeleev organize the entire table during his research?
the elements showed no periodic properties.
the atomic number was unknown at the time.
some elements were undiscovered at the time.
the atomic mass was unknown at the time.

Explanation:

Mendeleev organized elements by atomic mass and noticed periodic trends. But many elements hadn't been discovered in his era, preventing a complete periodic - table organization.

Answer:

C. Some elements were undiscovered at the time