Which type of structure is ideally used for a laser backstop?

An opaque wall or hill is ideally used for a laser backstop because it effectively prevents the laser beam from passing beyond its intended target area. The purpose of a laser backstop is to absorb or deflect the laser energy in order to prevent it from posing a hazard to any individuals or property beyond the designated shooting area. An opaque structure ensures that the laser light does not transmit through or escape, significantly enhancing safety in training environments where lasers may be employed.

Unlike the other options, which may allow some form of light penetration, an opaque wall or hill provides a solid barrier that mitigates the risk of unintended exposure to the laser. Glass barriers or transparent windows could allow light to pass through, potentially creating hazardous situations. Safety nets, while useful in other contexts, do not serve the specific function of containing laser energy effectively.