The missing words for the blank spaces are the same word which is:
Coenzymes.
This means that the one way they can accomplish to regulate the body is to serve as coenzymes. Thus, all Vitamin B are coenzymes.
Coenzymes Vitamins allow enzymes to perform their important work of facilitating metabolism in the body. Metabolism is the chemical process that occurs within a living organism in order to maintain life.
A is the correct answer. Cultural diversity fosters an openness for minority rights (e.g. Immigrant rights)
I'm pretty sure it's both A and D, but the more correct answer is D.