Step 1: Understanding quartiles.
For 80 employees, each quartile has \( \tfrac{80}{4} = 20 \) salaries. The first quartile consists of the lowest 20 salaries. Mark’s salary is the second-highest in this quartile, meaning it is the 19th salary overall.
Step 2: After hiring 8 new employees.
The company will then have 88 employees. Each quartile will now contain \( \tfrac{88}{4} = 22 \) salaries. The 22 lowest salaries will be in the first quartile, and the next 22 in the second quartile.
Step 3: Position of Mark’s salary.
Mark’s original position was 19th. With 8 new employees added at the bottom, his position becomes \( 19 + 8 = 27 \).
Step 4: Quartile placement.
- Quartile 1: positions 1–22
- Quartile 2: positions 23–44
Mark’s salary is now at position 27, which lies in Quartile 2.
Step 5: Relative position in Quartile 2.
In Quartile 2, Mark’s salary is \( 27 - 22 = 5 \)-th lowest. Thus, it is the third-lowest salary in the second quartile.
Step 6: Conclusion.
The correct answer is:
\[
\boxed{\text{(D) The third-lowest salary in the second quartile.}}
\]