The correct option is D Mollusca
Organisms using general body surface as their organs of gaseous exchange belong to phylums Porifera, Cnidaria, Platyhelminthes, Aschelminthes and Annelida. Amphitrite, though an annelid, uses external gills as the organs of gaseous exchange.
In molluscs, feather-like gills are present inside the mantle cavity called ctenidia which help in gaseous exchange.