In the modern periodic table, the value of quantum number is indicated by the period.

The correct option is C Principle

Just take a look at the periodic table, you will understand it.

If we write the electronic configurations of elements from different periods, they always start with the next number, indicating the increase in principal quantum number.

We can observe that the period number indicates the average energy level of orbitals or number of shells present in the atom and we know that principal quantum number (n) represent the shells in an atom. Hence period in the periodic table indicates the Principal quantum number (n) i.e. number of shells.