Ribbon-shaped chloroplast is the characteristic feature of

The correct option is Spirogyra.

Let us understand the detailed biological explanation for this characteristic feature step-by-step:

1. What is Spirogyra?
Spirogyra is a genus of filamentous, multicellular green algae belonging to the order Zygnematales. It is commonly found in freshwater environments and is easily recognized by its unique cellular structure under a microscope.

2. Nature of Chloroplasts in Spirogyra:
The most defining and characteristic feature of Spirogyra is the presence of ribbon-shaped (or spiral-shaped) chloroplasts. Unlike many other algae that have cup-shaped, star-shaped, or reticulate chloroplasts, the chloroplasts in Spirogyra cells are arranged in a helical or spiral ribbon form along the periphery of the cytoplasm. This spiral arrangement is what gives the genus its name "Spirogyra".

3. Comparing with other options:
- Riccia: This is a bryophyte (liverwort), not green algae. Its cells contain small, disc-shaped (discoid) chloroplasts standard for land plants.
- Chlamydomonas: This is a unicellular green alga which typically possesses a single, large cup-shaped chloroplast.
- Ulothrix: This is a filamentous green alga which is characterized by a girdle-shaped (or collar-shaped) chloroplast that partially encircles the cell.

Therefore, ribbon-shaped chloroplasts are specifically characteristic of Spirogyra.