The blind sac body plan is the body plan where the there is a single opening of the digestive system. The tube-like digestive system starts with a single opening through which the food is taken inside and the digested food is expelled through the same opening. The gut is lined with a single layer of endodermal cells that absorb and digest food. Cnidaria and Platyhelminthes exhibit this type of body plan.So, the correct answer is option C.