I see 2 options for this. Like 1.x I would select the post then choose the approve option from the drop-down. This doesn't work anymore. What does work is the "approve" button above the thread. Is this intended because it's confusing.
The post itself isn't awaiting approval -- the thread itself is. That's what the button at the top changes. It's essentially the same parallel between deleting a thread and individual posts. (XF1 generally would've had similar behavior.)
