I believe the correct answer is C. a sense of comfort and security.
This is not just any kind of overtaking or conforming or keeping within limits. It means those five nations will function as one, protect their common interests along with protecting each other, and prosper together. Also, the togetherness will make them far more secure. Hence the word "enfold", which also means to wrap up, to hug, to embrace.
I would say " as sense of comfort and security to keep within its limits" sounds like the most likely connotation of the word "enfolded" in the context of the explanation of the complete union of the 5 nations, "united into one head, one body and one mind".
