Final answer:
The correct signal to listen for while investigating the incidents involving the homeless person suspected of lighting fires would be D: 10-31, which indicates a crime in progress or suspect in action.
Step-by-step explanation:
The question being asked pertains to which signal to listen for while investigating a string of incidents involving a homeless person suspected of lighting trash fires in subway cars in the Bronx.
The correct signal to listen for in this context would most likely be D: 10-31. In law enforcement code, '10-31' generally means crime in progress or suspect in action, which in this case would apply to the act of lighting fires. The 10-28 codes are generally used for vehicle registration information, which would not be directly relevant to this situation.