Assertion (A) The elements having $1{\mathrm{s}}^2,2{\mathrm{s}}^2,2{\mathrm{p}}^6,3{\mathrm{s}}^2\text{ and }1{\mathrm{s}}^2,2{\mathrm{s}}^2$ configuration belong to same period.
Reason (R) Both have same number of outermost shells.

$2{\mathrm{p}}^6,3{\mathrm{s}}^2⟶ \mathrm{Belongs} \mathrm{to} 3\mathrm{rd} \mathrm{period}$

$1{\mathrm{s}}^2,2{\mathrm{s}}^2$$⟶$Belongs to 2ndperiod

Both have ${\mathrm{ns}}^2$,valence shell electronic configuration, so both have same number of outermost shell.