The answer is: c.enrolling native children in boarding schools
Assimilation refers to the process of absorbing a dominant culture and integrate it with minority's culture. Enrolling native children in boarding schools would increase the socialization that they do with White Children so they can observe each other's culture and adapt.
The Preamble is part of the Deceleration of Independence, and I'm pretty sure that preserving the articles of confederation wasn't part of the preamble.