From the following, the least stable structure is:

To determine the stability of resonating structures, we follow these rules:
1. Structures with more covalent bonds are generally more stable.
2. Structures where all atoms have complete octets are more stable.
3. Separation of opposite charges decreases stability, while like charges on adjacent atoms highly destabilize the structure.
4. Negative charge on a more electronegative atom and positive charge on a more electropositive atom is preferred.
In structure (4), there are two positive charges ($+$) located on adjacent atoms (the carbon atom and the nitrogen atom of the nitro group).
Electrostatic repulsion between like charges on adjacent atoms makes the structure highly unstable.
In structures (1), (2), and (3), like charges are not on adjacent atoms.
Therefore, structure (4) is the least stable.