Which of the following terms best describes organisms that require complex organic substances for nutrition and are often multicellular?

The term that describes organisms requiring complex organic substances for nutrition and that are often multicellular is fungi. Fungi are heterotrophic organisms, meaning they obtain their food by absorbing organic compounds from their environment, often breaking down complex substances. They play an essential role in the ecosystem as decomposers, breaking down organic material and recycling nutrients.

Fungi can be unicellular, like yeasts, but many are multicellular, such as molds and mushrooms. Their multicellular structure often involves hyphae, which form a network called mycelium that helps in nutrient absorption. This characteristic highlights their complex nutritional needs compared to autotrophs, which synthesize their nutrients from inorganic sources like sunlight.

Helminths, while also multicellular and heterotrophic, are specifically parasitic worms, which limits their definition in terms of nutritional requirements. Prokaryotes, which include bacteria and archaea, are often unicellular and can be autotrophic or heterotrophic, making them unsuitable as a description for primarily multicellular organisms. Therefore, fungi best fit the criteria of requiring complex organic substances and often being multicellular.