A scientist notices that the sky is covered by a thick layer of clouds. What kind of cloud is he MOST LIKELY seeing?

its called a cumulus cloud

Explanation:

white, puffy clouds that look like pieces of floating cotton. Cumulus clouds are often called "fair-weather clouds". The base of each cloud is flat and the top of each cloud has rounded towers. When the top of the cumulus clouds resemble the head of a cauliflower, it is called cumulus or towering cumulus. These clouds grow upward and they can develop into giant cumulonimbus clouds, which are thunderstorm clouds.

stratocumulus clouds

Explanation: Thick, dense stratus or stratocumulus clouds producing steady rain or snow often are referred to as nimbostratus clouds. ... A cumulus cloud that exhibits significant vertical development (but is not yet a thunderstorm) is called cumulus congestus or towering cumulus.