Final answer:
Mendeleev predicted the existence and properties of yet-to-be-discovered elements, which was confirmed with the discovery of gallium and germanium, known as eka-aluminum and eka-silicon. His work on the periodic table became widely accepted after these elements were found.
Step-by-step explanation:
The question relates to Dmitri Mendeleev's contribution to the field of chemistry and the development of the periodic table. Mendeleev predicted the discovery of several elements based on the patterns he observed in the properties of known elements. The answer to the question is B) elements. Mendeleev had organized elements in a table according to their atomic mass and noticed that the properties of elements showed a recurring pattern, which is known as periodicity. He also left gaps in his periodic table, predicting the existence and properties of elements that had not yet been discovered, which included eka-aluminum and eka-silicon. These predictions were spectacularly confirmed with the later discoveries of gallium and germanium, whose properties closely matched Mendeleev’s forecasts, solidifying the acceptance of his periodic table within the scientific community.
Notably, element number 101, discovered much later, is named mendelevium in honor of Mendeleev's work. The correct organization of elements, despite some exceptions that Mendeleev made due to atomic masses, was crucial in the recognition of the value of his periodic table. While Mendeleev had to adjust some of the positions of elements to align them with similar properties in the table, his intuition proved correct and the fundamental reasons behind these properties were understood later with the development of atomic theory.