Answer: a. acoelomates
Explanation:
Flatworms belong to the phylum Platyhelminthes. The body of these worms are flattened from top to bottom, that’s why these worms are called flatworms. They are bilaterally symmetrical and triploblastic (having three germ layers) organisms but they lack a body cavity or coelom. The body systems such as digestive, muscular, nervous, excretory, and reproductive systems are developed. Examples are flukes, tapeworms.