Answer
$A\cap B \cap C = \left\{a\right\}$
Work Step by Step
RECALL:
$\cap$ represents the intersection of set sets.
The intersection of two or more sets is the set that contains all the elements that are common to all the sets involved.
Notice that the element that is common to the sets A, B, and C is $a$.
Thus,
$A\cap B \cap C = \left\{a\right\}$
