Why did Mendeleev leave some gaps in his periodic table of elements?

The elements are listed in Mendeleev's periodic table in ascending order of relative atomic mass. He saw a periodic tendency in the chemical characteristics of the existing elements as he organised the atoms.
Russian scientist Mendeleev proposed a table diagram of the known elements in 1869. Only 63 elements were known at the time. He puts the elements in ascending order of atomic mass. The most intriguing feature of Mendeleev's periodic table, however, is that he left certain empty spots for elements that had not yet been identified and anticipated their atomic masses and chemical characteristics.

The elements were grouped by Mendeleev into groups and eras. The elements are organised in ascending order of atomic mass when stacked left to right in a row. He realised that if he first placed the eight items in each row, followed by the elements from the subsequent row, the columns would include elements with comparable qualities. He gave the groupings of columns names. The rows of Mendeleev's periodic chart are known as the term periods.
Mendeleev, for instance, foresaw the element that would be lacking from row 5 of group 3. Mendeleev also predicted that the missing element will have an atomic number of 68 and be a soft metal. Gallium is the name of the element that scientists discovered. And in this manner, scientists discovered all the missing elements that Mendeleev had anticipated.

Hence, the correct answer is option A.