Which of the following molecule(s) is/are having $\mathrm{p\pi }-\mathrm{p\pi }$ back bonding?

Since, This type of bonding takes place when one atom in a compound has a single pair of electrons and the other has empty orbitals close to one another. Pi-bonding features are present in a compound with back bonding because it happens after the formation of a sigma bond. Back bonding has an impact on the bond angle. So, molecule(s) is/are having $\mathrm{p\pi }-\mathrm{p\pi }$ back bonding.

Hence, the correct option is A, B,C, D.