Answer
Option $B$
Work Step by Step
Given two sets $A$ and $U$, so that $A\subseteq U$, the complement of $A$ is the set of all elements in $U$ that are not in $A$.
Based on the explanation above,
$A^c = \left\{5, 6, 7, 8\right\}$.
Thus, the answer is Option B.
