The first five ionization energies of an element are 9.1, 16.2, 24.5, 35 and 205.7 eV respectively then, the number of valence electrons in the atom is :

The first five ionization energies of an element are 9.1, 16.2, 24.5, 35 and 205.7 eV respectively then, the number of valence electrons in the atom is 4.

Since $I{E}_4$ and $I{E}_5$have a significant IE difference, the value of $I{E}_5$is quite high in comparison. Getting an atom's $5^{th}$ electron out is challenging. Because of this, it had to be in the last shell. It consequently has 4 valence electrons in the outermost shell. The element achieves noble gas structure following the elimination of 4 electrons.

Hence, option C is correct.