Final answer:
The Roman Catholic Church was the primary group commissioning Mannerist and Baroque art during the 1500s, using it as a tool in the Counter-Reformation to convey powerful religious messages.
Step-by-step explanation:
The group that most frequently commissioned Mannerist and Baroque art during the 1500s was the Roman Catholic Church. This period witnessed the transition from the structured principles of the High Renaissance to the more emotional and exaggerated forms of Mannerism, and subsequently into the dramatic and powerful style of the Baroque, heavily influenced by the Counter-Reformation. The Church employed art as a tool in the fight against Protestant Reformation, leading to a surge in commissions that sought to inspire, educate, and leave an impression on believers. During the Baroque period, artists like Caravaggio, Gentileschi, and Bernini created works under the patronage of the Church that emphasized emotional intensity and were often associated with religious themes.