Which unicellular organism acquires food by a process in which cells absorb external material by engulfing it with the cell membrane?

Amoeba

The correct answer is Option (3) - Amoeba

The unicellular organism that acquires food by a process in which cells absorb external material by engulfing it with the cell membrane is Amoeba.

Amoeba is a type of protozoan that belongs to the phylum Rhizopoda. It is characterized by its shapeless, flexible cell membrane, allowing it to change its form through a process called amoeboid movement. Amoebas primarily live in freshwater environments, and they obtain their food through a feeding process called phagocytosis.

Phagocytosis: Amoebas feed by extending their pseudopodia (temporary projections of the cell membrane and cytoplasm). When the amoeba encounters a food particle, it surrounds and engulfs it by forming a temporary pocket called a food vacuole. This process is known as phagocytosis.

Amoebas are heterotrophic organisms, meaning they rely on external sources for their food. This method of obtaining nutrients is different from autotrophic organisms, such as plants, which can produce their own food through photosynthesis. Amoebas are found in various aquatic environments and play a crucial role in the microbial ecosystem as predators of bacteria and other small organisms.