Oddly enough the Donna moment for this meme was easier to pick than either the Rose or the Martha moments. And it wasn't for lack of awesome moments! It's just that I immediately went to a particular scene and knew it was the one.
Donna Sharing the Responsibility in Fires of Pompeii
Donna spends much of the episode Not Really Getting It when it comes to what Ten can and can't do to change the time line. Or rather it's not that she doesn't get it; it's that she is unwilling to simply accept his pronouncements without some explanations, especially when so many lives are on the line. However, when it comes down to it and it is Pompeii versus the Earth, she's ready to share the responsibility and support him completely.