Explanation : A delocalized π bond are those π bonds in which the electrons are allowed to move freely over more than two nuclei. So, amongst the given molecules is the correct answer. As is a molecule which shares a π bond between one carbon and three oxygen atom. The electrons move freely over the whole molecule.
You should remember that a pi bond is related to an alkene, compounds that have a double bond. Among the choices, the only compound that has a double bond is the CO3^2-. Since it is an ion, the pi electrons are delocalized, meaning, their location cannot be indicated as they rotate around the molecules. This is a result of following the octet rule.
