Final answer:
The social isolation of Mexican Americans and Puerto Ricans was historically caused by government actions and societal discrimination, and not by choice or insignificance(option a).
Step-by-step explanation:
The social isolation of Mexican Americans and Puerto Ricans was a result of a combination of government policies and societal discrimination(option a).
Throughout American history, these communities faced numerous challenges such as being outvoted by an influx of Anglo-American settlers, facing discrimination in employment, and being relegated to segregated neighborhoods. This isolation was further exacerbated by government-sanctioned racism which illegitimately targeted all minorities, and the prevalence of corrupt state and local governments that often favored white people in land disputes and other matters.