Which tool is NOT commonly associated with adjusting opacity?

The Rectangle Tool is primarily used for creating shapes and does not directly modify or interact with the opacity of layers or selections in design software. When working on tasks related to adjusting opacity, other tools typically come into play. For example, the Blur Tool can soften the edges of elements, which can give an impression of varying opacity. The Clone Stamp can replicate parts of an image, and while it does not adjust opacity directly, it may be used in conjunction with layer adjustments that change transparency. The Eraser also directly interacts with the underlying layers and can modify their appearance, including creating effects related to opacity. Thus, the Rectangle Tool does not have a built-in function for adjusting opacity, making it distinct from the other tools mentioned.