Odd one among the following:

The odd one among the following compounds is azulene.

Explanation:
To determine the odd one among the given compounds, let's analyze the structures and properties of each compound.

Indene:
- Indene is a polycyclic aromatic hydrocarbon with a fused benzene ring and a cyclopentadiene ring.
- It has a total of 9 carbon atoms.
- The structure of indene is relatively stable and it exhibits aromaticity.
- It is commonly used as a building block in the synthesis of various organic compounds.

Anthracene:
- Anthracene is also a polycyclic aromatic hydrocarbon with three fused benzene rings.
- It has a total of 14 carbon atoms.
- Similar to indene, anthracene is stable and exhibits aromaticity.
- It is often used as a precursor for the synthesis of dyes and other organic compounds.

o,m,p-Xylene:
- o,m,p-Xylene refers to a mixture of three isomers, namely ortho-xylene, meta-xylene, and para-xylene.
- These isomers have the same molecular formula (C8H10) but differ in the arrangement of their methyl groups.
- They are colorless, flammable liquids that are commonly used as solvents and in the production of various chemicals.

Azulene:
- Azulene is a bicyclic organic compound with a fused cyclopentadiene and cycloheptatriene ring system.
- It has a total of 10 carbon atoms.
- Unlike indene, anthracene, and xylene, azulene does not exhibit aromaticity due to the presence of a bridged carbon atom.
- Azulene is known for its blue color and is often used as a dye and in the synthesis of pharmaceuticals.

Conclusion:
Among the given compounds, azulene is the odd one as it does not possess a fully aromatic structure like indene, anthracene, and xylene. Its bridged carbon atom disrupts the aromaticity, making it structurally different from the other compounds.