Final answer:
Rena's family could only remain in Krakow during WWII through external aid from resistance figures like Irena Sendler or Raoul Wallenberg; however, remaining hidden was extremely perilous.
Step-by-step explanation:
The only way Rena's family could remain in Krakow under the Nazi occupation during World War II was to seek help from individuals like Irena Sendler or through organized efforts such as those arranged by Raoul Wallenberg.
Sendler saved 2,500 Jewish children by smuggling them out of the Warsaw Ghetto, while Wallenberg provided Jews with protective passports and established safe houses in Budapest.
Unfortunately, as the case of the Frank family and many others illustrates, staying in Krakow without being discovered and deported by the Nazi regime was incredibly risky and nearly impossible.