Final answer:
The appearance of thousands of missionaries from developing countries has been referred to as the most significant step in mission history since Carey's voyage to India, reflecting a transformative era in global missionary dynamics.
Step-by-step explanation:
The event that has been called "the most significant step in mission history since Carey sailed for India in 1793" is (a) The appearance of thousands of missionaries from developing countries. This movement marks a remarkable shift in the dynamics of global missionary work, with a larger involvement of nations that were previously on the receiving end of mission efforts. The surge of missionaries from these countries represents a substantial change in the landscape of international religious outreach and signifies a departure from traditional histories where predominantly Western countries sent missionaries to convert peoples in Africa, Asia, and the Pacific.
The history of missionary work includes various significant milestones such as the endeavor by Catholic countries like Portugal, Spain, and France to spread Catholicism in their colonies, and the Protestant missionary work during the era of imperialism where Christian ethics and American virtues were fervently promoted. This evolution of mission work culminated in what is considered today as a highly significant step in mission history, reflecting the changing geopolitics and spiritual leadership in the modern world.