I’m not an expert on the field, but based off of process of elimination and what I do know, I’d say b.
In a 2 rescuer Basic Life Support for a child with an unresponsive child with cardiac arrest, activation of the Emergency Response system occurs initially after 2 minutes of CPR as one rescuer should be with the victim and doing the CPR while the other initiates the Emergency Response retrieving the AED. The compression-to-ventilation ratio is 15 is to 2.