Answer:
B. Cumulus: narrow, smoky
Step-by-step explanation:
The "cumulus" clouds are cotton-like, thus they are not supposed to be narrow nor smoky. They're usually described as having a "fluffy" appearance.
The description of "cirrus" clouds is correct. It is indeed wispy and feathery; thus, it is located at high elevation.
The description of "cumulonimbus" clouds is correct. Having a flat top means that it is at its "mature stage."
The description of "stratus" clouds is also correct. It is indeed made of flat layers; thus it is located at low levels.