Answer
$A \cap C'= \left\{b, c, d\right\}$
Work Step by Step
RECALL:
(1) The complement of set A, represented by $A'$, is the set that contains all the elements of the universal set $U$ that are not elements of $A$.
(2) $\cap$ represents intersection of sets. The intersection of two sets is a set that contains all the elements that are common to the two sets.
To find the elements of the given set, you have to (i) find the complement of $C$ then (ii) find the intersection of A and $C'$ afterwards.
Note that:
$C'=\left\{b, c, d, f\right\}$
Thus,
$A \cap C'= \left\{a, b, c, d\right\} \cap \left\{b, c, d, f\right\} = \left\{b, c, d\right\}$
