Which type of organism often requires an intermediate host in its life cycle?

Many types of parasitic organisms, including cestodes, trematodes, and nematodes, can have complex life cycles that involve multiple stages and may require one or more intermediate hosts to complete their development.

Cestodes, or tapeworms, typically have a life cycle that includes at least one intermediate host, often a vertebrate, where larval stages develop before transitioning to the definitive host where they mature and reproduce.

Trematodes, or flukes, also generally require intermediate hosts, usually snails, for their larval stages. The adult fluke matures in a definitive host, which can be various vertebrates, including humans.

Nematodes, or roundworms, can have diverse reproductive strategies. Some species require intermediate hosts for certain stages, while others do not. However, it's common for many nematodes that are parasitic to have at least an intermediate stage that might depend on another organism for development.

Given that all these groups often involve intermediate hosts in their life cycles, stating that all of them require this in their life cycle is valid. Thus, choosing the option that encompasses all these organisms correctly recognizes the common feature of requiring intermediate hosts in life cycles across various classes of parasites.