The correct answer is A) Hebrews.
The people that were the firsts to willingly accept the idea of worshipping only one god were the Hebrews.
The Hebrew people were the first to adopt the belief in one God. Indeed, it was Patriarch Abraham of Father Abraham, who instilled the idea of one God. Years later, in Mount Sinai, God gave the Ten Tablets or Ten Commandments to Moses. The First commandment says "You shall have no other Gods but me."