The figure of speech used in the phrase 'and whirls the mills' is:

The phrase 'and whirls the mills' refers to the wind, which is given the human-like ability to whirl/turn mills. Earlier lines also personify the wind as 'It walks on the water, and... talks to itself on the tops of the hills.' Attributing human actions (walking, talking, whirling) to the wind is personification. Hence option (4), Personification, is correct.