Which verb describes the movement of a flag or clothes on clothes line? [closed]

Solution 1:

Wave would be the most general verb.

to move freely and gently back and forth or up and down, as by the action of air currents, sea swells, etc.: flags waving in the wind.

^{http://www.thefreedictionary.com/wave}

Other than that, as mentioned in the comments also, there are the verbs flutter, fly, flap, billow, ripple, snap, flaunt and stream that you can use depending on the context or depending on the strength of the wind.

You can see this comparison in Google Ngram also:

Solution 2:

If you want to say what the wind does to the flag, rather than what the flag does in the wind, use flutters (light wind), whips (strong fast wind), ruffles (gentle irregular wind), and search synonyms for those -- just make sure they are verbs that can take a direct object (the wind does X to Y, not just the wind does X)