What Is Meant by "Rebuttable Presumption" by Courts in Regards

What is meant by "rebuttable presumption" by courts in regards to child custody?

A) The court will give custody to the mother unless the father can document that she is "unfit."
B) The court will mandate joint custody unless the parents can prove that joint custody is not advisable or appropriate.
C) The court will give custody to the parent chosen by the child if the child presents acceptable reasoning.
D) The court will give primary custody to the parents who was the primary caretaker and secondary custody to the other parent with guaranteed frequent visits and consultation on decisions impacting the child.