Reindeer moss is a lichen known as

The correct option is Cladonia.

Reindeer moss is not actually a true moss, but rather a species of fruticose lichen. Its scientific name is Cladonia rangiferina, which belongs to the genus Cladonia and the family Cladoniaceae.
Lichens are composite organisms consisting of a symbiotic relationship between a fungus (the mycobiont) and an algae or cyanobacterium (the photobiont).

Let's look at why the other options are incorrect:
- Usnea: Commonly known as beard lichen or old man's beard, it is a genus of fruticose lichens that grow on trees.
- Rocella: Known for yielding litmus dye, it is a genus of lichenized fungi in the family Roccellaceae.
- Parmelia: A genus of foliose lichens with a leafy structure, often found on rocks and tree bark.

Therefore, Reindeer moss is scientifically classified under the genus Cladonia.