Personally, I prefer "What do you think about extracting this into a separate function?" Unlike in any of your examples, the subtext is: "I anticipate you had

Personally, I prefer "What do you think about extracting this into a separate function?" Unlike in any of your examples, the subtext is: "I anticipate you had reasons for doing it the way you did and I am open to listening to them." It engages the author on the same footing, leaves the door open for discussion but still communicates your intent in a straightforward manner.