Animals: Invertebrates
A body plan is a set of morphological characteristics that are shared by the majority of the members of a phylum. It includes a specific symmetry, certain layers, the segmentation type, and the disposition of nerves, limbs, and gut.
A sac body plan refers to the presence of a gut cavity that has only one opening, which serves both the functions of the mouth and anus.
Some groups with this body plan are Cnidaria (jellyfish), Porifera (sponges), and Platyhelminthes (flatworms).
So, of the options, only Annelida DOES NOT have a sac body plan, given that its members have a mouth and an anus. So A is the correct option.