- Hexane is a straight-chain or branched-chain alkane with the formula \(C_6H_{14}\), and it can have several structural isomers.
- Option (1) is a straight-chain alkane with 6 carbon atoms, so it is an isomer of hexane.
- Option (3) is another form of hexane, with a different branching, and hence it is also an isomer of hexane.
- Option (4) is another valid structure that corresponds to an isomer of hexane.
- Option (2) shows a cyclic structure, which is not an isomer of hexane. This compound is a ring-chain isomer of hexene and not of hexane.
Therefore, the correct answer is:
\[
\text{(2) is not an isomer of hexane.}
\]