Final answer:
Quorum sensing in bacteria is initiated when the cell population reaches a certain density, allowing for coordinated biological activities via the release and binding of molecules called autoinducers. So, the correct option is b) The cell population reaches a certain threshold.
Step-by-step explanation:
Quorum sensing is triggered to begin when the cell population reaches a certain threshold. This biological process, predominantly studied in bacterial populations, is a method where microorganisms are able to detect their own density in an environment by releasing and binding small, diffusible molecules known as autoinducers. The presence of a quorum is essential for certain behaviors to be coordinated within a bacterial community. These behaviors, such as the production of virulence factors in pathogens, are beneficial only when there are enough cells present to significantly impact their surroundings, whether it be to overwhelm a host's immune defenses or in other symbiotic relationships.