To effectively remove stains from the interior pool surface, what action is required?

To effectively remove stains from the interior pool surface, draining the pool and performing an acid wash is often necessary. This process involves draining the water, which allows for direct access to the stained surfaces. An acid wash helps to remove mineral buildup, organic stains, and other deposits that can accumulate over time, restoring the pool's appearance. Acid washing is particularly effective on plaster surfaces and is a common practice when stains are persistent and cannot be removed with regular cleaning methods.

The other options, while they may contribute to overall pool maintenance, do not directly address stain removal from the pool's interior. Regular backwashing of filters is essential for maintaining water clarity and filtration efficiency, but it does not directly influence stain removal from surfaces. Raising the pH of pool water to 8.0 is not a recognized method for removing stains and could potentially complicate water chemistry, leading to additional challenges in pool maintenance. Thus, the most effective option for stain removal is to drain and acid wash the pool.