Which of the following organisms consists of unicellular cells joined to form a filament?

Analyze the question: We need an organism whose body is made of cells joined end to end in a thread-like chain (a filament).

Explain the concept: Some algae are unicellular, some are colonial, and some are filamentous. A filamentous alga has many cells attached one after another in an unbranched (or sometimes branched) thread.

Go through the options:

• A. Amoeba: A single-celled protozoan that moves by pseudopodia. It lives alone and does not form filaments.
• B. Spirogyra: A green alga ("pond silk" or "water silk") with cylindrical cells joined end to end in long, unbranched filaments. Each cell has a distinctive spiral (ribbon-like) chloroplast. This fits the question.
• C. Euglena: A single-celled organism with a flagellum and an eyespot. It lives alone.
• D. Chlamydomonas: A single-celled green alga with two flagella. It is free-swimming and not filamentous.

Answer: Spirogyra is the only one whose cells join to form a filament, so the correct option is B.

Tip: Remember "Spirogyra = Spiral chloroplast + String-like filament." Both start with S. Also remember that Amoeba, Euglena and Chlamydomonas are all solitary single cells.