Ectoparasites differ from endoparasites in that they:

The correct answer is Option (3) → Feed on the external surface of the host organism.

Parasites are grouped by where they live on the host. Ectoparasites feed on the external surface of the host, such as lice on humans, ticks on dogs and copepods on marine fish. Endoparasites live inside the host body, in the liver, kidney, lungs or red blood cells.

Option (1) Live inside the host body at different sites. — Incorrect. Living inside the host body at different sites is the feature of endoparasites.

Option (2) Obtain their nutrition only from the host plant they parasitise. — Incorrect. This describes a parasitic plant such as Cuscuta, which takes its food from the host plant. Most ectoparasites (lice, ticks) live on animals, so this is not what sets them apart.

Option (3) Feed on the external surface of the host organism. — Correct. Ectoparasites stay outside the host and feed on its surface.

Option (4) Have more complex life cycles. — Incorrect. It is the endoparasites that have more complex life cycles, because of their extreme specialisation.

NCERT Extract:
Population Interactions – “… Parasites that feed on the external surface of the host organism are called ectoparasites. The most familiar examples of this group are the lice on humans and ticks on dogs. Many marine fish are infested with ectoparasitic copepods. Cuscuta, a parasitic plant that is commonly found growing on hedge plants, has lost its chlorophyll and leaves in the course of evolution. It derives its nutrition from the host plant which it parasitises. … In contrast, endoparasites are those that live inside the host body at different sites (liver, kidney, lungs, red blood cells, etc.). The life cycles of endoparasites are more complex because of their extreme specialisation.”