The number of elements known at the time of Russian chemist Dmitri Ivanovich Mendeleev was:

Correct option is C

The correct answer is (c) 63
At the time of Dmitri Ivanovich Mendeleev, there were 63 known elements. Mendeleev is renowned for formulating the Periodic Table, which he developed in 1869. This table organized the 63 elements known at the time according to their atomic mass and properties, laying the groundwork for the modern periodic table where elements are organized by increasing atomic number.
Mendeleev's periodic table had gaps for elements that had not yet been discovered. He was able to predict the properties of these missing elements, such as gallium, scandium, and germanium, with remarkable accuracy. This prediction capability demonstrated the effectiveness of his table and provided strong evidence for its validity.
The table arranged the elements into rows and columns in such a way that elements with similar properties fell into the same column, known as groups. This arrangement highlighted the periodic recurrence of elements' properties, which is why the table is known as the "periodic" table.