In the sentence "This helped sustain Guarani," the best replacement for "This" is C. This system
Isolation: While isolation can sometimes protect a culture, it's unlikely to be the sole factor sustaining it in the long term. Guarani communities have interacted with other groups throughout history.
Territory: While having a defined territory can be important for cultural survival, it's not the only factor at play. The Guarani have adapted and thrived in various environments.
Colonization: Colonization generally hurts indigenous cultures, so it wouldn't be considered something that sustains them.
System: This is the most fitting option as it refers to the broader social, economic, and cultural structures that support the Guarani way of life. This could include their agricultural practices, social organization, language, and spiritual beliefs.
Therefore, "This system" best captures the idea of the underlying structures and practices that have helped the Guarani people endure and maintain their cultural identity.