Which of the following compounds are also known as aliphatic compounds?

Acyclic compounds

The correct answer is option 2. Acyclic compounds.

The compounds known as aliphatic compounds are Acyclic compounds

Aliphatic compounds are organic compounds that do not contain a benzene ring. They can be either acyclic (open-chain) or cyclic (closed-chain). Acyclic compounds have straight or branched chains of carbon atoms and include substances such as alkanes, alkenes, alkynes, and organic acids. In contrast, aromatic compounds (option 1) contain one or more benzene rings. Cyclic compounds (option 3) include both aliphatic and aromatic cyclic structures. Benzenoid compounds (option 4) are a subset of aromatic compounds that specifically refer to compounds derived from benzene.