Which of the following is NOT one of the four main groups of clouds?

The answer is correct because "nebulous" is not recognized as one of the four main groups of clouds. The four main types of cloud classifications typically include cirrus, cumulus, stratus, and nimbus. Each of these classifications helps describe the appearance and formation of clouds.

Cumulonimbus refers to a specific type of cumulus cloud that can develop into severe thunderstorms, characterized by towering structures and associated with precipitation. Stratiform clouds refer to layers of clouds such as stratus clouds that can cover the sky in broad sheets. Cumuliform refers to a cloud type characterized by its puffy, white appearance with a flat base – commonly associated with fair weather cumulus clouds.

" Nebulous," on the other hand, is a term more often used in various contexts to describe vague or indistinct concepts rather than a specific type of cloud. Therefore, it does not fit within the recognized categories for cloud classification in meteorology.