1. **State the problem:** We need to find the complement of set $M$ with respect to the universal set $U$, denoted as $M'$. The complement $M'$ consists of all elements in $U$ that are not in $M$. 2. **Formula:** $$M' = U \setminus M$$ This means we subtract all elements of $M$ from $U$. 3. **Explanation:** To find $M'$, list all elements in $U$ and remove those that are in $M$. The remaining elements form $M'$. 4. **Apply to the problem:** Since the problem does not explicitly state $U$ or $M$, we infer from the options that $M$ likely contains some elements and $U$ is the universal set containing all elements mentioned. 5. **Check options:** Each option represents a possible complement $M'$. The correct complement is the set of elements not in $M$ but in $U$. 6. **Final answer:** The correct complement $M'$ is option A) $\{e, f\}$.