Final answer:
The nurse should collaborate with the healthcare provider to adjust the insulin dosage for the breastfeeding mother with type 1 diabetes, as insulin needs may change postpartum.
Step-by-step explanation:
The correct action for a nurse when a breastfeeding mother with type 1 diabetes reports a decrease in insulin needs after giving birth is B) Collaborate with the healthcare provider to adjust insulin dosage. Since type 1 diabetes requires the administration of insulin due to the pancreas not producing sufficient insulin, it is essential to monitor and adjust insulin dosage based on current needs rather than ceasing it entirely or making dietary changes without medical advice. Blood sugar levels can vary due to hormonal changes postpartum and the physical demands of breastfeeding, hence a healthcare provider's input is necessary for the proper management of insulin therapy during this period.