Final answer:
Charles IX, son of Catherine de Medici and king of France, is most notably associated with the St. Bartholomew's Day Massacre of 1572, a significant event during the French Wars of Religion.
Step-by-step explanation:
The son of Catherine de Medici who became king of France as a boy and ordered the St. Bartholomew's Day Massacre of 1572 under the influence of his mother was Charles IX. The king in question had a reign marked by religious conflicts, notably between Catholics and Huguenots (French Protestants). During his tenure, religious violence escalated, culminating in the massacre where Catholics viciously attacked Protestants in Paris. This event was a part of the larger struggle during the French Wars of Religion, exemplified by the War of the Three Henrys—Henry III, Henry of Guise, and Henry of Navarre—where control of the kingdom was fiercely contested. Peace eventually returned to France when Henry IV, initially a Protestant who succeeded the throne after the war, converted to Catholicism and issued the Edict of Nantes, granting religious liberties to the Huguenots and bringing stability to the country.