I just took the quiz and can confirm that the right answer is "material moving downslope as a thick fluid".
Source/Explanation (from my textbook) (keywords that I think best match the right answer have been underlined by me):
<em>Flows are </em><u><em>mass movements of material </em></u><em>containing a large amount of water, which </em><u><em>move downslope as a thick fluid.</em></u><em> Flows that move quickly, called </em><u><em>mudflows</em></u><em>, are common in semiarid mountainous regions, such as parts of southern California. </em>
