Final answer:
Mr. Frank allowed Mr. Van Daan's family to move in out of humanity during the Nazi occupation, as they sought to avoid deportation to extermination camps. They were supported by Dutch friends in the underground resistance and hid together in the annex, a history detailed in Anne Frank's diary.
Step-by-step explanation:
Mr. Frank allowed Mr. Van Daan's family to move in as an act of compassion and solidarity in a time of extreme peril. During the brutal occupation by the Nazis, the Frank family had fled to the Netherlands for safety, only to face the Nazis' systematic deportation of Jews to extermination camps from occupied Europe. As the danger escalated and it became clear that Holland was no safe haven, the Franks, along with other Jewish families such as the Van Daans, were compelled to go into hiding to evade capture and almost certain death. Hiding in the annex, the Franks and Van Daans received support from Dutch friends involved in the underground resistance, which was a lifeline in those dire times.
Although their eventual discovery led to their deportation, the act of sheltering the Van Daans was a gesture of humanity within the broader context of the Holocaust. Anne Frank's diary, a poignant witness to these events, serves as a testament to the lives of those in hiding as well as the unassailable evidence of the horrors they faced.