Which of the following groups of elements, discovered later, could be placed in Mendeleev’s periodic table without disturbing the existing order of groups?

The noble gases like helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn) were not known when Mendeleev gave his periodic table. These gases were discovered later on because they are inert and are present in extremely low concentration in our atmosphere. But, it was the strength of Mendeleev’s periodic table that these gases, when discovered, could be placed in a new group called zero group, without disturbing the existing order. Halogens (F, Cl, Br, I), alkali metals (Li, Na, K, Rb, Cs) and alkaline earth metals (Be, Mg, Ca, Sr, Ba) were already discovered when Mendeleev gave his periodic table. So, they were already occupying specific positions in his periodic table.