Which of the following best describes invertebrates?

Invertebrates are best described as organisms without a backbone. This distinction is fundamental in biology, as it categorizes organisms based on the presence or absence of a vertebral column. Invertebrates comprise a vast group of animals, including species such as insects, arachnids, mollusks, and cnidarians, which do not possess a spinal column. This characteristic significantly influences their anatomy, physiology, and evolutionary adaptations.

The other options highlight different biological concepts. For instance, organisms with a backbone belong to the vertebrates category, which includes mammals, birds, reptiles, amphibians, and fish. The mention of photosynthesis pertains to autotrophic organisms, primarily plants and some algae, which can produce their own food using sunlight. Additionally, the idea that only multicellular organisms are invertebrates overlooks the fact that there are unicellular organisms, like certain protozoa, that do not fit into this category. Thus, the identification of invertebrates simply revolves around their lack of a backbone.