Which effect would likely be used for a fade-in transition in After Effects?

Using opacity animation is the most effective method for creating a fade-in transition in After Effects. This technique involves keyframing the opacity property of a layer over time. Starting from an opacity of 0% (completely transparent), you can animate it to reach 100% (fully opaque), thus achieving a smooth and visually appealing fade-in effect.

During the animation process, you can customize the timing and easing options to create a more natural transition. This flexibility allows for adjustments in how quickly or slowly the fade occurs, enhancing the overall visual storytelling of your project.

While there are other effects that may suggest a similar outcome, they do not provide the straightforward control and simplicity offered by directly animating opacity. For instance, the dissolve effect is primarily used for creating a texture or a granular look and does not serve as an ideal solution for a smooth transition. Transition effects may involve more complex setups and are not specifically designed for just fading in layers. Similarly, a fade effect could be misconstrued as a generic term and may not denote a specific, practical application in the context of After Effects. Thus, animating the opacity is the most precise and effective choice for achieving the desired fade-in effect.