Most fungi are multicellular and also very susceptible to the environment, therefore D is incorrect.
Protista are mostly unicellular but they are also eukaryotic. Therefore, C is incorrect as well.
Both Archaebacteria and Eubacteria are groups of bacteria, but Archaebacteria are more primitive and are prokaryotic, while Eubacteria relates to eukaryotic cells. Therefore, B is wrong too.
Arachaebacteria can be found in harsh environments, are unicellular and prokaryotic. Therefore, the correct answer is A. Archaebacteria.