Answer:

Introduction:
Aromaticity refers to a property of certain organic compounds that have a ring structure and exhibit exceptional stability due to the delocalization of electrons. Aromatic compounds typically possess a cyclic, planar structure with a conjugated system of π electrons.

Explanation:
Out of the given options, the compound that is aromatic is the cycloheptatrienyl cation (D).

Cyclopentadienyl cation (A):
- Cyclopentadienyl cation is a cyclic compound with a positive charge on one of the carbon atoms.
- It contains 5 carbon atoms, forming a conjugated system of π electrons.
- However, it does not fulfill the requirements for aromaticity as it does not have a planar structure and its conjugated system is not complete.

Cyclooctatetraene (B):
- Cyclooctatetraene is a cyclic compound with eight carbon atoms and a conjugated system of π electrons.
- However, it does not exhibit aromaticity because it is non-planar and its conjugated system is not complete.
- The compound can undergo facile addition reactions due to its non-aromatic nature.

Cycloheptariene (C):
- Cycloheptariene is a cyclic compound with seven carbon atoms.
- It does not exhibit aromaticity as it does not have a planar structure and its conjugated system is not complete.

Cycloheptatrienyl cation (D):
- Cycloheptatrienyl cation is a cyclic compound with seven carbon atoms and a positive charge on one of the carbon atoms.
- It possesses a planar structure and a complete conjugated system of π electrons.
- The compound fulfills the requirements for aromaticity and is therefore considered aromatic.

Conclusion:
Among the given options, the compound that is aromatic is the cycloheptatrienyl cation (D). It possesses a planar structure and a complete conjugated system of π electrons, leading to exceptional stability and aromaticity.

Cyclohepta trieny cation commonly called as tropylium ion is aromatic