Final answer:
Trematoda, Monogenean, and Cestoda share the evolutionary adaptation of parasitism.
Step-by-step explanation:
An evolutionary adaptation shared by Trematoda (flukes), Monogenean (ectoparasitic flatworms), and Cestoda (tapeworms) is their ability to parasitize their hosts. These flatworms have evolved various adaptations to survive and reproduce within their hosts. For example, trematodes have complex life cycles that involve a mollusk secondary host and a primary host where sexual reproduction occurs. Cestodes infect the digestive systems of their primary vertebrate hosts.