Which task aims to ensure the effectiveness of a change after it's implemented?

The task that specifically aims to ensure the effectiveness of a change after it has been implemented is testing. Testing occurs after a change is put into effect and involves validating that the change meets its intended objectives and operates as designed within the system. This is crucial for identifying any issues that may arise from the change, ensuring that it delivers the expected benefits without causing unintended disruptions.

While review, planning, and implementation all play significant roles in the change management process, they focus on different stages. Review typically relates to evaluating the change process or its outcomes but does not inherently include assessing effectiveness post-implementation. Planning involves the preparatory steps taken before a change is executed, and implementation refers to the actual execution of the planned change. Testing is uniquely geared towards confirming the success and stability of the change after it has taken place.