According to Mendeleev's periodic table, which unknown element of group III had an atomic mass of 68 that was later replaced by gallium ?

Eka-aluminium

The correct answer is option 3.Eka-aluminium.

Mendeleev's periodic table, proposed in 1869, organized the known chemical elements based on their atomic masses and chemical properties. When Dmitri Mendeleev constructed his periodic table, he left gaps for elements that were yet to be discovered. These hypothetical elements were referred to as "eka-elements," with prefixes indicating their position relative to known elements.

In the case of the element later identified as gallium, it was initially referred to as "eka-aluminium" (eka being a Sanskrit word meaning "one"), as it was predicted to have similar properties to aluminium but with a higher atomic mass. Mendeleev predicted the properties of this unknown element based on the trends observed in the periodic table.

Gallium (Ga), which was discovered in 1875 by the French chemist Paul-Émile Lecoq de Boisbaudran, filled the gap Mendeleev had left for eka-aluminium. Gallium's properties closely matched those predicted by Mendeleev, validating the periodic table's predictive power and leading to the acceptance of Mendeleev's periodic law.

Therefore, eka-aluminium was the hypothetical element with an atomic mass of 68 that was later replaced by gallium when it was discovered. This discovery further solidified the success and reliability of Mendeleev's periodic table in predicting the existence and properties of unknown elements.