Final answer:
Opening doors or windows improves visibility inside structures by allowing more natural light to enter, while the other options would generally not improve visibility and could in fact decrease it.
Step-by-step explanation:
The question you've asked relates to how visibility inside structures, such as buildings or homes, can be improved so that interior crews or inhabitants can work or live more effectively. Improving visibility is typically associated with increasing the amount of light. More specifically, the action that may improve visibility inside structures would be d) Opening doors or windows to allow more natural light to enter. This is much like how on a sunny day, big windows and glass doors let in a lot of light, improving visibility and helping interior crews to better carry out their work.
It's important to note that increasing humidity, turning off lights, and closing windows are not actions that would normally improve visibility. In fact, increasing humidity might lead to condensation on windows and surfaces, reducing visibility. Likewise, turning off lights will make a space darker, and closing windows could block natural light from entering.