How does shelley convey the idea that humans are constantly moving and changing? by personifying the moon by comparing humans to clouds by personifying the night by comparing humans to darkness

The correct choice is clouds in this case, because those have the closest connotation in literature with change. The elements given have several connotations in literature. For example, darkness implies that something is unknown or it refers to a sense of being lost.  Pesonification is a means of adding emotion and differing points of view to a story. The moon and the night that are treated there have similar connotations to darkness; the moon can also be said to wane or be a full moon so it encompasses change. HOWEVER, the most obvious choice is that clouds mean change; they always drift along and they change, appearing and disappearing.

The cloud are the correct choice

Their. It agrees with the subject as well.