Esau sold his birthright to Jacob primarily because he was starving after a long hunt and found Jacob cooking food (specifically lentil stew). In a moment of hunger and desperation, Esau asked Jacob for some of the stew, and Jacob offered it to him in exchange for his birthright. This story is found in the book of Genesis in the Bible (Genesis 25:29-34). The correct answer is:
because Esau was starving from a long hunt and Jacob had food.