Final answer:
The main theme depicted in Ambrogio Lorenzetti's 'Peaceful City' fresco is prosperity and peace.
Step-by-step explanation:
The main theme depicted in Ambrogio Lorenzetti's 'Peaceful City' fresco is Prosperity and peace. The fresco is part of a larger series of paintings called 'Allegory of Good Government', which were commissioned for the Palazzo Pubblico in Siena, Italy. The 'Peaceful City' fresco specifically portrays a thriving urban environment with prosperous citizens engaged in various activities, symbolizing the effects of good governance and peaceful coexistence.