Answer:
When it is said that a person's upbringing is determined by nature, it means that the upbringing or training of a person/child is pre-determined by his genes and natural inclinations.
Ocassionally, nurture plays a huge part too in a person's upbringing as it is as important as nature if not more important.
This is because, if a child inherits quick temper from his parents, he is always at risk of easily losing his temper on someone or something. However, nurture can also help curtail that if there is an enabling environment where it is not allowed to thrive.