Pokémon Adventures: Diamond and Pearl/Platinum Vol 4

From the VIZ Media edition

Mysteries abound when a Psyduck and a Pokemon Researcher go missing. And Lady fights two epic Gym battles! If only Pearl and Diamond could stop fighting each other...! Then, what secret does Cyrus, leader of Team Galactic, seek in the ruins and relics of Celestic Town...? Plus, meet Carnivine, Floatzel, Magnezone, Drifblim, Mismagius, Bronzong and many more Sinnoh Pokemon friends!

Source: Anime News Network Encyclopedia