I have written some articles about this matter in the past, but I have not directly answered the above question.  To have a good understanding whose Cain's wife was, we have to answer these 3 questions: Was Adam the first man? Did Eve have any child when Cain left? What kind of people lived in Nod compared to Eden?  In order to answer the above questions, let us look into the Bible (NIV) and let the characters in it speak for themselves.  

First, we analyze the dialogue between God and Cain (Genesis 4:10-16): After Cain kills his brother, Abel, God punishes him.  God basically tells him that he can no longer grow food as a farmer and he will become a wanderer.  Cain then begs God not to send him away because THEY will kill him.  God does not tell Cain that he is daydreaming.  Even God AGREES with Cain that THEY will kill him, so God gives Cain a mark.  Immediately after Cain gets the mark, he feels satisfied with his deal and quietly moves away.  The actual punishment for Cain in killing Abel was death at the hands of strangers.  Instead of asking for forgiveness, Cain basically asked God for a lesser judgment.  If Cain was the first born of all creation, why was he so scared of his younger distant siblings?  Could it be that THEY were a different ethnic group all together?  Well, God gave Cain a mark that appeared to put the mind of Cain to rest.   This must have been an artistic tribal mark uniquely for the people of Nod that would identify Cain as one of them.  The fear of Cain and the agreement of God about other peoples that even made God give Cain a mark of protection is evidence that these people were no descendants of Adam.  So Adam is not the first man even by Biblical account.  He may be the first prophet that was placed in the Garden of Eden in Genesis 2.  The Biblical age of Adam is about 8,000 years old, while the man of creation in Genesis 1, according to science and human fossils, may be 4.4 million years old. 

Third, we must compare the Land of Nod to Eden (Genesis 4:17-22) : In Eden, Adam and his family were mainly wearing leaves, living in tents, farming, following livestock around, and basically nomadic and rural in nature.  In Nod, they were building cities, making bronze and iron for tools or weapons, marrying many women, naming property after their children (legalized inheritance), and providing musical entertainment for occasions.  Clearly, Nod was more sophisticated than Eden as a society.  While Eden was still very rural, Nod was already a legal and service oriented society.  It was like comparing New York City to a village in the third world.   Eden may have been very innocent, but Nod was materialistic and very worldly.  If not for the mark of protection God gave Cain, Cain would have been killed in Nod and would not have been able to cope with their way of life.  They accepted Cain as their own and he became a builder instead of a farmer.  This is also evidence that Adam has not the father of the people of Nod and Cain's wife was a daughter of Nod.  If Cain, the supposed first born of all humans, was scared to death of the Nod people, what daughter of Eve would risk her life for a crime she did not commit by going with him? 

I strongly recommend that all Nigerians and Africans who love and know God to read a book titled "The Political and Spiritual Purpose of the Holy Land".  This book goes into details in answering these questions and it teaches you how to read the Bible the way God wanted us to read it and how to use God's word to genuinely improve your life and that of your nation spiritually, politically, economically, and common-sensually.   

The Political and Spiritual Purpose of the Holy Land: https://edofolks.com/political