Answer: Option (c) is the correct answer.
Explanation:
When we move across a period in a periodic table then there is increase in atomic mass of the atoms. This is due to increase in number of electrons due to which there occurs increase in number of shells down the group.
Whereas in a period, electrons are mostly added into the same sub-shell as size of atom decreases on moving from left to right in a period.
For example, atomic number of Li is 3 and in K shell it has 2 electrons, in L shell it has 1 electron.
Atomic number of boron is 5 and in K shell it has 2 electrons, in L shell it has 3 electrons.
Therefore, we can conclude that within any period we find elements with the same number of energy levels.
