Which of the following correctly defines the opacity adjustment?

The correct definition of opacity adjustment is that it determines to what degree a layer hides or shows layers beneath. This means that when you adjust the opacity of a layer, you are controlling how transparent or opaque that layer is in relation to the layers that are situated below it.

When a layer has 100% opacity, it is fully opaque, meaning it completely obscures any content behind it. Conversely, a layer with 0% opacity is completely transparent, allowing everything beneath to be fully visible. Adjusting the opacity value between these two extremes allows you to create a range of effects, such as blending two layers together or showing underlying textures or images through a top layer.

This functionality is central to many editing techniques in Photoshop, such as creating overlays, achieving softer transitions in compositing, or enhancing specific visual elements without losing the underlying context.