Final answer:
The most accurate statement about the Spanish mission system in California is that many California Indians who worked at the missions died from European diseases, as they lacked immunity to illnesses such as smallpox and influenza.
Step-by-step explanation:
Among the options provided regarding the truth about the Spanish California mission system, the statement that many California Indians who labored at the missions died of European diseases is the most accurate (option B). The Spanish colonial systems, such as the encomienda and repartimiento, involved the exploitation of Native American labor. The missionaries tried to convert the natives, but this process often led to the imposition of forced labor in mines or plantations, which was intensively burdensome, leading some to prefer death. Due to the lack of immunity, Native Americans suffered greatly from European diseases like smallpox and influenza, resulting in high mortality rates. The arrival of the Spanish, along with their push for conversion and labor exploitation, significantly disrupted the lives and cultures of the Native Americans in California and throughout the Americas.