Which choice best describes a mask in animation?

A mask in animation is used to control the visibility of parts of layers in a way that reveals or conceals underlying elements. This technique allows animators to create complex visual effects by showing only certain portions of the layers below the mask. When a mask is applied to a layer, it acts as a stencil: areas where the mask is opaque will show the underlying layers, while areas where the mask is transparent remain hidden. This is particularly useful in creating dynamic animations where you want to draw attention to specific parts of a scene or transition elements in and out of view seamlessly.

The other options do not accurately describe the function of a mask. A graphic used to change colors globally refers to color adjustments rather than visibility control. A permanent filter would affect the entire animation or layer effects uniformly, which is distinct from masking that selectively edits visibility. A tool for resizing objects pertains to transformation functions rather than the revealing aspect of a mask. Thus, the description of a mask as a layer that reveals underlying layers captures its core purpose within animation effectively.