Final answer:
In a situation where a 9-month-old infant is found not breathing and rigor mortis is present, the correct action is to not attempt resuscitation and immediately notify the authorities for further investigation and to provide support to the family.
Step-by-step explanation:
When called to a private residence where a 9-month-old infant is found not breathing with rigor mortis present, it indicates that the baby has been deceased for some time, and the onset of rigor mortis is a clear sign that resuscitation will not be successful. The appropriate action in this tragic situation is to refrain from resuscitation efforts and instead, promptly notify the authorities so that they can conduct an investigation and handle the situation according to legal and medical protocols. Additionally, it is vital to provide support and care to the grieving family during this extremely difficult time.