Final answer:
The reticular activating system is responsible for awakening a mother to the sound of her crying baby, as it regulates wakefulness and prioritizes sensory information.
Step-by-step explanation:
The part of the brain that awakens a mother to the sound of her crying baby is d) Reticular activating system. This system is crucial for regulating wakefulness and sleep-wake transitions. It is part of the brainstem, connecting with the thalamus to filter the vast array of incoming sensory information and prioritize it. While the hypothalamus, amygdala, and hippocampus are all part of the limbic system and play significant roles in regulating emotions, stress responses, and memory, it is the reticular activating system that is primarily responsible for alertness and awakening in response to specific stimuli like a baby's cry.