How do you search for a group of contacts that have a completed task?
You should be able to pull a report from here:
Admin > Reports > Task Note Report
In the Task/Note report, edit the search criteria and select ‘Completed’
The the act of completing a task also achieves a goal in the campaign builder, then you could use the Campaign Goal Completion report - this would be particularly useful if you’re only looking for contacts who had a specific task completed.