Answer
$C\cap A=\{a\}$
Work Step by Step
$A=\{a,b,c\}$ $;$ $C=\{a,d,f,g\}$
$C\cap A$
The intersection of two sets is a set that includes the elements that belong to both sets.
The intersection of $A$ and $C$ has the elements that are included in $A$ and $C$, simultaneously:
$C\cap A=\{a\}$