This is a crutose lichen

The correct option is Rhizocarpon.

Lichens are symbiotic associations between a fungus (mycobiont) and an alga or cyanobacterium (photobiont). Based on their growth forms, lichens are broadly classified into three main categories:
1. Crustose lichens: These form a thin, flat crust that is closely appressed to the substrate (such as rocks or tree bark) over the entire lower surface, making them very difficult to remove without damaging the substrate.
2. Foliose lichens: These are leaf-like, flat, and lobed, attached to the substrate at specific points by root-like structures called rhizines (e.g., Peltigera).
3. Fruticose lichens: These have a shrubby or hair-like, branching growth form that grows upright or hangs down from branches (e.g., Usnea).

Let us examine the options provided:
- Rhizocarpon: Commonly known as the map lichen, it grows on siliceous rocks in mountainous regions and forms a flat, tightly adhered crust. This is a classic example of a crustose lichen.
- Peltigera: This genus is characterized by large, leaf-like, lobed thalli, making it a foliose lichen.
- Usnea: Commonly called beard lichen, it has a shrubby, hair-like appearance, classifying it as a fruticose lichen.

Therefore, Rhizocarpon is indeed a crustose lichen, making it the correct choice.