The correct answer is A. crustose lichen.
Crustose lichens are the first plants to colonize bare rock, and they play a vital role in the process of xerarch succession. They help to break down the rock and create a thin layer of soil, which other plants can then use to grow. Lichens also help to improve the water retention of the soil, which makes it more suitable for other plants.
Mosses are also pioneer species in xerarch succession, but they are not as effective at breaking down rock as crustose lichens. They are also more susceptible to drought and other environmental stresses.
Foliose lichens are more tolerant of drought than mosses, but they are not as effective at breaking down rock. They are also more susceptible to being blown away by the wind.
Shrubs are not pioneer species in xerarch succession. They require a more developed soil environment than crustose lichens, mosses, and foliose lichens can provide.
In conclusion, crustose lichens are the pioneers in xerarch succession because they are the first plants to colonize bare rock and they play a vital role in the process of creating a more suitable environment for other plants.