Final answer:
The post that supports the handrail at its termination on an interior stair system is called a newel post, which is critical for both safety and design.
Step-by-step explanation:
On an interior stair system, the post that supports the handrail at its termination is called a newel post. A newel post is a structural element that is often decorative and provides critical support to the handrail, which is an important safety feature as well as a design element of a staircase.
The other options provided, such as baluster, stringer, and riser, refer to different parts of a staircase. A baluster is a vertical moulding that supports the handrail, a stringer is the structural member that supports the treads and risers of the staircase, and a riser is the vertical portion between each tread on the stair.
A newel post is the post that supports the handrail at the end of an interior stair system.
On an interior stair system, the post that supports the handrail at its termination is called a newel post.