Which of the following is a classic example of a single celled (unicellular) organism?

Introduction / Context:
Living organisms can be single celled (unicellular) or multicellular. Single celled organisms carry out all life processes within one cell, while multicellular organisms use many specialised cells organised into tissues and organs. Exam questions often ask you to recognise examples of unicellular organisms among a mix of plant, animal, and protist examples. This question tests that basic classification skill.

Given Data / Assumptions:

• You are asked to identify a single celled organism.
• Options include Spirogyra, Amoeba, Hydra, and Earthworm.
• We assume basic school level knowledge of algae, protozoa, and simple animals.

Concept / Approach:
A unicellular organism consists of just one cell that performs all essential functions such as feeding, movement, reproduction, and excretion. Amoeba is a classic example of a unicellular protozoan that lives in water or moist soil. It moves using pseudopodia and engulfs food particles. Spirogyra is a filamentous green alga made up of many similar cells joined end to end. Hydra and Earthworm are multicellular animals with tissues and organs, far more complex than a single cell.

Step-by-Step Solution:
Step 1: Recall that Amoeba is commonly shown in textbooks as a single celled organism with an irregular shape and pseudopodia. Step 2: Recognise that Spirogyra forms long filaments composed of many cells and is therefore multicellular, even though each cell looks similar. Step 3: Recall that Hydra is a small freshwater animal with a tubular body and tentacles, clearly multicellular. Step 4: Remember that an Earthworm is a large segmented invertebrate with specialised organs, making it multicellular. Therefore, the only classic unicellular example in the list is Amoeba.

Verification / Alternative check:
Classification tables group Amoeba with protozoa, which are typically unicellular eukaryotes. Microscopic examinations show Amoeba as a single cell without fixed shape that divides by binary fission. In contrast, Spirogyra is described as an alga that forms long chains of cells, Hydra is placed in the phylum Cnidaria, and Earthworm in the phylum Annelida, both clearly multicellular. These classification facts confirm that Amoeba is the correct choice.

Why Other Options Are Wrong:

• Spirogyra: Multicellular alga composed of many cells arranged in filaments, not a single cell.
• Hydra: A multicellular animal with tissues and simple organs; it is not unicellular.
• Earthworm: A complex multicellular organism with many organ systems, including digestive and circulatory systems.

Common Pitfalls:
Some learners assume that any microscopic organism or simple looking plant must be unicellular. However, many algae and simple animals are multicellular even if they are small. It is useful to memorise a few standard unicellular examples such as Amoeba, Paramecium, and certain bacteria, and to recognise that larger visible organisms like worms and hydroids are almost always multicellular.

Final Answer:
A classic example of a single celled (unicellular) organism is Amoeba, a free living protozoan.