The correct answer is B) Native American peace and acceptance.
What motivated Spanish missionaries to strengthen their commitment to convert the plains Indians to Christianity was "Native American peace and acceptance."
Spaniards, during the colonial period, had many objectives in mind when they got to the North American territory. Spaniard's missions wanted to evangelize the Native Indians into the Catholic religion. Missionaries also wanted to establish peace in the territories for a better colonization process, and they wanted to educate the Indians to the Spanish culture and customs.