To solve this problem, we need to identify which group of organisms is found in extreme saline conditions.$\newline$Let's analyze each option:$\newline$• Option 1: Eubacteria - These are typical bacteria found in various environments, but not specifically in extreme saline conditions.$\newline$• Option 2: Cyanobacteria - These are photosynthetic bacteria, commonly found in aquatic environments, but not necessarily in extreme saline conditions.$\newline$• Option 3: Mycobacteria - These are a type of bacteria known for causing diseases like tuberculosis, and are not associated with extreme saline environments.$\newline$• Option 4: Archaebacteria - These are a group of microorganisms that are known for living in extreme environments, including high salinity.$\newline$Archaebacteria are well-adapted to survive in extreme conditions such as high salt concentrations, high temperatures, and acidic or alkaline environments.$\newline$Therefore, the correct answer is Option 4: Archaebacteria.