Single-celled animals are classified as

Living organisms are classified by cell count into unicellular (single-celled) forms and multicellular (many-celled) forms. A unicellular organism carries out every life process — nutrition, movement, respiration, reproduction — within one single cell. In the five-kingdom classification, eukaryotic unicellular organisms are grouped as Protozoa, placed under Kingdom Protista precisely because their entire body is one cell.

The question describes animals whose body is a single cell. By the definition above, that description matches only Protozoans — organisms such as Amoeba and Paramecium, whose entire body is one cell that performs all life functions on its own.

• Arthropods (e.g. insects, spiders, crabs) have a segmented body built of many specialised cells, protected by a jointed exoskeleton — not a single-cell body plan.

• Mammals (e.g. dogs, whales, humans) are vertebrates whose organs and tissues are built from trillions of differentiated cells — not a single-cell body plan.

• Molluscs (e.g. snails, clams, octopuses) have a soft body of many specialised cells, often enclosed in a shell — not a single-cell body plan.

Under the Whittaker five-kingdom system, Protozoa is the only one of these four groups placed in Kingdom Protista on the basis of a one-cell body; Arthropods, Mammals, and Molluscs are all members of Kingdom Animalia, which by definition comprises multicellular organisms. This confirms Protozoans as the correct classification for single-celled animals.