An example of organism which exists as a colony is

1. Analyze the question The question asks which organism lives as a colony, meaning a group of individual cells living together as one organized unit, usually with some cooperation between the cells.

2. Explain the concept

• Unicellular organisms live as single, independent cells.
• Colonial organisms are made of many cells of the same type living together. Each cell can often survive on its own, but the cells cooperate. Some show a simple division of labour.

3. Go through the options

• A. Volvox: A green alga that forms a hollow, spherical colony of hundreds to thousands of flagellated cells. The cells beat their flagella together to roll the colony through the water. Some cells are specialized for reproduction. It is the classic textbook example of a colonial organism.
• B. Paramecium: A single-celled, slipper-shaped protozoan. It is not colonial.
• C. Euglena: A single-celled, flagellated organism with both plant-like and animal-like features. It is not colonial.
• D. Chlamydomonas: A single-celled green alga with two flagella. It is not colonial, although it is closely related to Volvox.

4. Answer Volvox is the colonial organism, so the correct option is A.

5. Exam tip Remember the phrase "Volvox = Volume of cells": it is the big ball of many cells. Other colonial examples that often appear in exams are Pandorina and Spirogyra (filamentous). Also remember that Paramecium, Euglena and Chlamydomonas are all single-celled.