Answer: D: on a mountain top.
Step-by-step explanation:
A watershed is a section of land that gathers precipitation in different areas, like streams or tributaries, and drains them out to larger waterways, like oceans or rivers.
Because watersheds are inherently considered areas of LAND, it cannot be option B or C.
Similarly, water is unlikely to flow from the bottom of a valley, so it, too, cannot be a watershed.
However, water could flow from a mountain top down to a river or ocean.