Answer:
Here, the stimulus which is discussed in the question is the Suckling stimulus which is also known as Suckling reflex.
Suckling by a child stimulates the suckling reflex system. The stimulus (information) reaches to the Hypothalamus. Hypothalamus triggers the TRH (thyrotropin-releasing hormone) which further triggers the releases of the Prolactin (PRL) hormone by the Anterior Pituitary.
Finally, when Prolactin reaches the blood, it stimulates the Mammary glands which being stimulated, start to produce milk.
Explanation:
The main function of Prolactin is to produce milk. Prolactin production is mainly controlled by Hypothalamus. Prolactin hormone is produced in Pituitary, uterus, etc. The prolactin hormone production is also controlled by the estrogen.