Which of the following is NOT a feature of aliphatic compounds?

The characteristic feature that distinguishes aliphatic compounds is that they can consist of open-chain structures, which may be straight or branched, and may also be either saturated or unsaturated. When discussing cyclic compounds, aliphatic structures refer specifically to acyclic (non-cyclic) forms. As a result, while some cyclic compounds can fall under the category of alicyclic compounds, these are not considered true aliphatic compounds because they can exhibit aromatic properties or behaviors similar to aromatic compounds. Thus, option B, which states that they can be cyclic, is the correct choice for something that is not a feature of aliphatic compounds.

The other features correctly describe aliphatic compounds: they indeed can have straight or branched chains, do not contain aromatic rings, and can also be either saturated or unsaturated. This reinforces the concept of aliphatic compounds being distinctively non-cyclic and primarily open-chain in structure.