Given below is the electronic configuration of elements A, B, C, D.

a. A and B are the elements coming within the same period.

i. As the total no of valence electrons (electrons in the outermost shell) decides the period number of the element.

ii. A (1s² 2s²) and B (1s² 2s² 2p⁶ 3s²) both have two valence electrons.

Hence, they belong the same period, i.e., second period.

b. B and C are the elements coming within the same group.

⇒ As the highest principal quantum(n) number decides the group number of the element.

⇒ In B (1s² 2s² 2p⁶ 3s²) and C (1s² 2s² 2p⁶ 3s² 3p³), 3 is the highest quantum number.

⇒ Hence B and C belong to the same group, i.e., third group

c. D is the noble gas element

⇒ D has atomic number 10.

⇒ This means 2 in K-shell and 8 in the L-shell which is a complete

Noble gas configuration.

d. C belongs to 5^th period and 3^rd group.

⇒ C has 5 electrons in the outermost shell (3s² 3p³). Hence it belongs to 5^th period.

⇒ In C (1s² 2s² 2p⁶ 3s² 3p³), 3 is the highest quantum number. Hence, it belongs to third period.