When A and B are the wave functions of atomic orbitals, then is represented by:

🧠 In MO theory, combining two atomic wave functions gives two molecular orbitals:

• Bonding MO: $\sigma = \psi_A + \psi_B$ (constructive interference, electron density increases between nuclei)
• Antibonding MO: $\sigma^* = \psi_A - \psi_B$ (destructive interference, electron density decreases/creates a node between nuclei)

The antibonding $\sigma^*$ MO is represented by the difference of the two atomic wave functions.

⚡ The − sign introduces a nodal plane between the nuclei (where $\psi_A = \psi_B$ and the wave function becomes zero), which is the defining feature of an antibonding MO.

$\boxed{\text{Answer: (b) } \psi_A - \psi_B}$