What property must be animated to create a fade-in effect on a layer?

To create a fade-in effect on a layer, animating the opacity property is essential. The opacity parameter controls the transparency of a layer, with a value of 0% being fully transparent and 100% being fully opaque. By keyframing the opacity from 0% to 100%, you can smoothly transition the layer from invisibility to full visibility, which is the essence of a fade-in effect.

This technique is widely used in video production and motion graphics to enhance the visual appeal by allowing elements to gradually appear on the screen, rather than appearing abruptly. Opacity animations are straightforward and effective for achieving various visual effects, including fades in and out.

In contrast, other properties such as position, scale, and rotation manipulate the layer's physical location, size, or orientation in space, but they do not affect its visibility directly. Therefore, they do not create a fade-in effect; instead, they may create movement, resizing, or spinning animations, which serve different purposes in a composition.