What method is used to combine multiple layers into a single layer in After Effects?

The method used to combine multiple layers into a single layer in After Effects is known as pre-composition. Pre-composing is an essential tool that allows users to group selected layers into a new composition, which can then be treated as a single layer in the parent composition. This is particularly useful for organizing complex projects, enabling you to apply effects or transformations to multiple layers at once, streamline your workflow, and maintain a cleaner timeline.

When layers are pre-composed, all the selected layers become part of a new composition. This new composition can also be independently edited, providing flexibility for any adjustments without affecting the original parent layers directly. It can encapsulate animations, effects, or layering arrangements, making it easier to manage intricate projects.

The other methods mentioned are either not standard practices for combining layers in After Effects or refer to different processes. For instance, layer merging would imply a more permanent combination that might not allow for separate edits afterward. Composition grouping isn't a recognized terminology in After Effects, and layer nesting is often used interchangeably with pre-composition but technically refers to how layers are used within compositions rather than merging them into a singular entity.