Which best describes a mudflow?

sudden movement along a flat, inclined surface

movement too slow to be observed directly

material moving downslope as a thick fluid

material falling freely through the air

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):

Flows are mass movements of material containing a large amount of water, which move downslope as a thick fluid. Flows that move quickly, called mudflows, are common in semiarid mountainous regions, such as parts of southern California.

3 looks like the best option