Final answer:
Joseph's primary duty in the context given, which conflates different biblical narratives, would most likely be to follow the instruction from an angel to go to Egypt due to a king's decree.
Step-by-step explanation:
According to the given information, what is Joseph's primary duty? Based on the biblical chronology, after Joseph's family joined him in Egypt, they faced increasingly poor treatment, leading up to the pharaoh's decree to kill all male Hebrew children. However, the information provided does not directly relate to Joseph's duties but rather to the general historical context of the Hebrews in Egypt. The question may be confusing aspects of different timelines, as the Joseph referred to in the biblical account of the Egyptian sojourn is not directly connected with later events such as the birth of Moses or the proclamation of the Ten Commandments.
Nonetheless, considering the broader narrative of the Hebrews in Egypt, the closest answer that can be deduced is option c) To follow instructions from an angel and go to Egypt due to a king's decree. This aligns with the biblical account where Joseph, not to be confused with the later figure of Moses, is warned by an angel to escape to Egypt to evade King Herod's decree in the New Testament narrative, though this is separate from the earlier Joseph's experience in Egypt.