Which automation tool would you select if the goal is to take action based on defined criteria in a one-time operation?

The most suitable choice for taking action based on defined criteria in a one-time operation is a segmentation rule.

Segmentation rules allow you to create a specific group of contacts or leads that meet certain criteria, and this action can be executed once to identify the desired segments within your database. These rules are particularly effective in scenarios where you want to group contacts based on specific attributes or behaviors at a particular point in time, facilitating targeted campaigns or analyses.

In contrast, automation rules typically involve ongoing processes and regular checks, which may not align with a one-time operation. Completion actions are tied to specific actions taken by a user within a system, triggering subsequent actions in a workflow that may not fit the one-time criteria intended here. Dynamic lists, while useful for grouping contacts based on ongoing criteria, do not specifically execute an action in the manner defined in the question.