Riccia is a genus of liverworts, which are small, non-vascular plants that belong to the division Marchantiophyta. Liverworts, including Riccia, have a simple thallus structure, which consists of a flat, ribbon-like body. The thallus is composed of cells and may have different types of scales that provide protection and aid in various functions. Among the options provided, the correct type of scales found in Riccia is multicelled and ligulate.

Explanation:
1. Liverwort structure:
Liverworts, including Riccia, have a thallus structure, which is a flattened body that lacks true roots, stems, and leaves. The thallus is composed of cells and can vary in shape and size.

2. Scales in Riccia:
Scales are small structures found on the surface of the thallus in liverworts. They serve various functions, including protection against desiccation, herbivory, and UV radiation.

3. Multicelled scales:
Multicelled scales are composed of multiple cells. These scales can be found on the thallus of Riccia. The multicelled scales in Riccia are typically elongated and narrow, resembling a tongue or strap-like structure.

4. Ligulate scales:
Ligulate scales are scales that are tongue-like or strap-like in shape. They are often elongated and narrow. These scales can be found on the thallus of Riccia.

5. Unicelled scales:
Unicelled scales are scales composed of a single cell. These scales are not found in Riccia.

Conclusion:
In summary, the correct type of scales found in Riccia is multicelled and ligulate. These scales are elongated and narrow, resembling a tongue or strap-like structure. They serve various functions, including protection against desiccation and herbivory.