1. Identify the key features of the molecular orbital diagram

   To classify a molecular orbital, we look for:

   • whether the overlap is head-on or sidewise
   • whether there is electron density between the nuclei
   • whether there is a node between the nuclei

2. Distinguish between $\sigma$ and $\pi$ orbitals

   • A $\sigma$ orbital is formed by head-on overlap along the internuclear axis.
   • A $\pi$ orbital is formed by sidewise overlap of orbitals, with electron density above and below the internuclear axis.

   Since the diagram is of a molecular orbital with sidewise character, it must be a $\pi$-type orbital, not a $\sigma$ orbital.

3. Distinguish between bonding and antibonding

   • In a bonding orbital, electron density is concentrated between the two nuclei.
   • In an antibonding orbital, there is a node between the nuclei, so electron density is absent from the internuclear region.

   The diagram indicates destructive overlap with a node between the nuclei, so it is antibonding.

4. Combine both observations

   • Type of overlap: $\pi$
   • Nature: antibonding

   Therefore, the orbital is an antibonding $\pi$ orbital, written as $\pi^*$.

5. Check options

   • A: A non-bonding orbital $\rightarrow$ Incorrect
   • B: An antibonding $\sigma$ orbital $\rightarrow$ Incorrect
   • C: A bonding $\pi$ orbital $\rightarrow$ Incorrect
   • D: An antibonding $\pi$ orbital $\rightarrow$ Correct

Hence, the correct answer is D.