The atomic number of $Na$ and $Mg$ is $11$ and $12$ respectively and they belong to the same period.
$(a)$ Which one would have smaller atomic size?
$(b)$ Which one would be more electropositive?
$(c)$ To which group would each one belong?

(A-D) Magnesium $(Mg)$ has a smaller atomic size than Sodium $(Na)$. As we move from left to right across a period,the nuclear charge increases,which pulls the electrons closer to the nucleus,resulting in a decrease in atomic radius.
$(b)$ Sodium $(Na)$ is more electropositive than Magnesium $(Mg)$. Electropositivity is the tendency of an atom to lose electrons. Since $Na$ has a lower nuclear charge and a larger atomic size,it can lose its valence electron more easily than $Mg$.
$(c)$ Sodium $(Na)$ belongs to Group $1$ (Alkali metals),and Magnesium $(Mg)$ belongs to Group $2$ (Alkaline earth metals).