Which of the following organic compounds is a benzenoid?

The correct answer is Aniline.

To understand why aniline is a benzenoid organic compound, let's break down the classification of organic compounds based on their structures:
1. Acyclic or Open-Chain Compounds: These compounds contain open chains of carbon atoms. Examples include propane and hexane.
2. Cyclic or Closed-Chain Compounds: These compounds contain one or more closed rings of atoms. These are further classified into:
- Alicyclic Compounds: Ring compounds that behave like aliphatic (open-chain) compounds.
- Aromatic Compounds: Special cyclic compounds that possess stability due to a conjugated system of double bonds obeying Huckel's rule. Aromatic compounds are further divided into:
- Benzenoid Aromatic Compounds: Aromatic compounds that contain one or more benzene rings in their structure.
- Non-benzenoid Aromatic Compounds: Aromatic compounds that do not contain a benzene ring (e.g., tropolone).

Let's analyze each of the given options:
- Acetaldehyde: It has the chemical formula CH₃CHO. It is an open-chain (acyclic) aldehyde, not a cyclic or aromatic compound.
- Propane: It has the formula CH₃CH₂CH₃. It is a straight-chain (acyclic) alkane.
- Hexane: It has the formula CH₃(CH₂)₄CH₃. It is also a straight-chain (acyclic) alkane.
- Aniline: It has the formula C₆H₅NH₂. Its structure consists of a benzene ring (a six-membered carbon ring with alternating double bonds) attached to an amino group (-NH₂). Because it contains a benzene ring, aniline is classified as a benzenoid aromatic compound.