Name one ‘single cell’ which behaves like a complete organism.

Hint: The organism which behaves like a complete organism despite being a singular cell is Amoeba. Amoeba as we know, is a unicellular organism. It has the ability to change its body shape to an extension, in order to engulf the food particles. Amoeba do not form a single taxonomic group; instead they are found in every major lineage of eukaryotic organisms.

Complete answer:
Amoeba is an important Protozoa which is found in freshwater. Amoeba intake on microscopic plants and animals present in water. And the process by which it obtains food is called phagocytosis.
Amoeba is a unicellular animal, so it does not have a mouth for inserting food. Amoeba gulp the food by encircling it with the help of pseudopodia. When the food is completely encapsulated from every side, the food is engulfed in the bag called food vacuole.
There are steps in the food mechanism in Amoeba- ingestion, digestion, absorption, assimilation and excretion.
1. Ingestion- ingestion is the first step and process of intake of food.
2. Digestion- digestion is the process of breaking large and unbreakable molecules into small and water-soluble molecules. In Amoeba, several digestive enzymes get mixed up into the food and break the food into simpler ones.
3. Absorption- the food digested by digestive enzymes is being absorbed in the cytoplasm by the process called diffusion.
4. Assimilation- the food which is absorbed by the cytoplasm is used to obtain energy, growth and for repair of cells. This process of utilizing food for obtaining energy is called assimilation.
5. Egestion- when a sufficient amount of undigested food gets accumulated inside a food vacuole, it is thrown out of the body by rupturing cell membrane.

Note:
Other charactericstics in Amoeba are reproduction, Amoeba reproduced through asexual mode of reproduction, Specially by binary fission. Binary fission is the type of reproduction in which the Amoeba cuts its halves and those halves develop into new individuals. Amoeba has a high rate of reproduction ability.