Which characteristic defines helminths?

Helminths are defined as parasitic worms, which is why this choice is the correct answer. These organisms belong to a diverse group of invertebrates that can include flatworms (such as cestodes or tapeworms) and roundworms (such as nematodes). Helminths typically inhabit the gastrointestinal tract of their hosts, but they can also be found in other tissues. What distinguishes them is their complex life cycles, which often involve multiple hosts and developmental stages.

The other options do not accurately describe helminths. For instance, helminths are not single-celled organisms; this characteristic applies to protozoa. Additionally, they are not beneficial microbes, as most helminths are known for their parasitic lifestyles, causing diseases in their hosts rather than providing benefits. Lastly, while some helminths may have life stages that involve external parasitism, they are predominantly classified as internal parasites, making the option relating to external parasites inaccurate. Overall, the defining characteristic of helminths is their status as parasitic worms, affecting a wide range of animal hosts.