The correct answer is a) all have bilateral symmetry.
Bilateral symmetry is a characteristic where the body can be divided into two identical halves along one plane. This feature is common in flatworms (such as planarians), molluscs (like snails and octopuses), and annelids (like earthworms). Each of these groups exhibits this type of symmetry, which is indicative of more complex organism structures compared to those with radial symmetry or asymmetry.
Options b, c, and d are incorrect. Flatworms are indeed acoelomates, but that characteristic does not apply to molluscs and annelids, which are coelomates. Parazoans do not exhibit bilateral symmetry; they are characterized by a lack of true tissues, as seen in sponges. Lastly, vertebrates are a subgroup of animals different from flatworms, molluscs, and annelids, which are invertebrates. Thus, the defining characteristic linking flatworms, molluscs, and annelids is bilateral symmetry.