Final answer:
Bacteria capable of thriving at -15°C can include species from Proteobacteria, Cyanobacteria, and Gram-positive bacteria. Archaea, though not bacteria, also have members that can survive in extremely cold environments. The correct options are A, B, and C.
Step-by-step explanation:
Bacteria that can thrive at extremely low temperatures, such as -15°C, are often referred to as psychrophiles. In the list provided, Proteobacteria, Cyanobacteria, and Gram-positive bacteria can all include species capable of living in such cold environments.
Archaea, while not technically bacteria, also includes extremophiles capable of surviving in cold temperatures. Of note, the Cyanobacteria played a significant role in shaping the Earth's atmosphere by producing oxygen. Gram-positive bacteria, like Actinobacteria, are common and have the capability to withstand various extreme conditions.
On the other hand, Proteobacteria is a diverse phylum that includes many subgroups adaptable to a range of environments, including cold ones.