Answer
(a)
Work Step by Step
The alkali metals are in group 1 of the periodic table.
Option (b) is incorrect. Chlorine (Cl$_{2}$) is a halogen in group 17.
Option (c) is incorrect. Xenon (Xe) is a noble gas in group 18.
Option (d) is incorrect. Bromine (Br$_{2}$) is a halogen in group 17.
Option (a) is correct. Sodium (Na) is an alkali metal in group 1 of the periodic table.
