Aromatic compounds are:A)Open chain compoundsB)Closed chain compoundsC)Both open and closed chain compounds.D)Closed chain compounds which are structurally similar to benzene.

To solve the question about aromatic compounds, let's first understand what aromatic compounds are and how they are classified.

### Definition of Aromatic Compounds
Aromatic compounds are a class of cyclic (closed-chain) compounds that have specific structural characteristics. The most notable characteristic of aromatic compounds is that they contain one or more benzene rings, which are stable due to resonance.

### Key Characteristics of Aromatic Compounds
1. **Cyclic Structure**: Aromatic compounds are always cyclic (closed-chain).
2. **Planarity**: The structure must be planar, allowing for effective overlap of p-orbitals.
3. **Hückel's Rule**: They follow Hückel's rule, which states that a compound is aromatic if it has \(4n + 2\) π electrons (where \(n\) is a non-negative integer). For example, benzene, which has 6 π electrons, fits this rule with \(n = 1\).

### Analyzing the Options
Let's evaluate the options provided in the question:

- **A) Open chain compounds**: Aromatic compounds cannot be open-chain (acyclic). This option is incorrect.

- **B) Closed chain compounds**: This option is partially correct, as aromatic compounds are closed-chain; however, it does not specify that they are specifically similar to benzene, which is crucial.

- **C) Both open and closed chain compounds**: This is incorrect because aromatic compounds are exclusively closed-chain.

- **D) Closed chain compounds which are structurally similar to benzene**: This option is correct. Aromatic compounds are indeed closed-chain structures that share structural similarities with benzene, making this the best choice.

### Conclusion
The correct answer is **D) Closed chain compounds which are structurally similar to benzene**. Aromatic compounds are specifically cyclic structures that exhibit resonance and stability similar to that of benzene.