The correct option is
C Soredia
Lichens reproduce vegetatively by soredia formation. It is the most common form of reproduction where small protuberances are formed on the thallus. They may have a definite pustule or appear as greyish dust. Many lichens reproduce asexually, either by vegetative reproduction or through dispersal of diaspores, which contain a few algal cells surrounded by fungal cells.
Thus, the correct answer is option D.