The correct answer is option 3: 38th Parallel.

The boundary between North and South Korea is marked by the 38th Parallel. The 38th Parallel is a line of latitude that runs horizontally across the Korean Peninsula. It was established as part of the Korean Armistice Agreement signed in 1953 at the end of the Korean War. This line divides the Korean Peninsula into two separate countries, North Korea and South Korea.

Option 1, 16th Parallel, is incorrect. There is no significant boundary or division between North and South Korea marked by the 16th Parallel.

Option 2, 29th Parallel, is also incorrect. The 29th Parallel does not play a role in the division between North and South Korea.

Option 4, 49th Parallel, is incorrect. The 49th Parallel is the border between the United States and Canada, not between North and South Korea.

Therefore, the correct answer is option 3, 38th Parallel.