Answer
These signalling systems would be needed for interactions between and among prokaryotic cells, such as when they exchange DNA during conjugation and when they cooperate, as when cyanobacteria and other prokaryotes form biofilms.
Work Step by Step
The key to this answer is to see that cell signalling requires some interaction between different cells. Then, try to imagine when prokaryotic cells interact (skimming chapters on cell structure and prokaryotic diversity may help).
