The graph between the square root of the frequency of a specific line of characteristic spectrum of $X$-rays and the atomic number of the target will be

The spectrum obtained from a sodium vapour lamp is an example of

The solar spectrum is an example of

The ionisation energy of an electron in the ground state of a helium atom is $24.6 eV$. The energy required to remove both electrons is: (in $eV$)

Given below are two statements:
Statement $I$: Atoms are electrically neutral as they contain equal number of positive and negative charges.
Statement $II$: Atoms of each element are stable and emit their characteristic spectrum.
In the light of the above statements,choose the most appropriate answer from the options given below.

If elements with principal quantum number $n > 4$ were not allowed in nature,then the number of possible elements would be