Final answer:
In the SOAP format of charting, the 'P' domain stands for 'Plan'. The data that should be documented under the 'P' domain is 'Client referred to the health unit's drop-in breastfeeding clinic.'
Step-by-step explanation:
In the SOAP format of charting, the 'P' domain stands for 'Plan'. This domain is used to document the nurse's plan of action for the patient. Out of the options given, the data that should be documented under the 'P' domain is option d. 'Client referred to the health unit's drop-in breastfeeding clinic.' This information represents the nurse's plan to refer the client for further assistance.