Final answer:
Multicultural therapy challenges the assumption that Western-developed psychological theories are universally applicable, emphasizing the need to consider cultural context in psychological processes.
Step-by-step explanation:
The primary assumption about the nature of psychological processes that multicultural therapy attempts to challenge is the notion that certain psychological theories and practices, which have been predominantly researched and developed in Western, Educated, Industrialized, Rich, and Democratic (WEIRD) societies, are universally applicable to other, non-WEIRD populations. Multicultural therapy scrutinizes this assumption by asserting that cultural context and environmental factors greatly influence an individual's behavior, perception, and mental health. It highlights the importance of adapting psychological theories and practices to honor the unique psychological processes of individuals from diverse cultural backgrounds, rather than applying a one-size-fits-all approach.