For the Old Testament, the Septaguint, of course, as that is the one that Christ Himself (and his Apostles) used and taught from (and the one eviscerated by the post-Christian Jews, and which Martin Luther adopted). There is some disagreement on the New Testament, but certainly the canon of the New Testament was complete by ca. 400AD. The RC Bible is unchanged from that time to present day.
No historical proof for a BC Septuagint; probably 200 AD. Same error made by Protestant seminaries. No Orthodox temple or synagogue would use a Greek OT. Why would the Jews of Christ day use one?
But then claiming a BC Greek OT, you can only go back to 400 for your NT ?? That is because the Catholic NT came from Alexandria after 325 AD. It had been sitting being mutilated and corrupted by Origin and his ilk in N. Africa until Constantine ordered 50 copies of it, which ended up in Rome. Original? Not by a long shot. Byzantine/Antiochan NT copies were all over Asia Minor, along with a Latin OT (160 AD) long before that.