What does a packet’s header contain?

A packet's header indeed contains critical information necessary for the proper handling, routing, and reassembly of data as it traverses a network. The header typically includes the packet's source and destination addresses, sequence numbers, and protocols being used, all of which are essential for reassembling the data correctly at the destination.

When packets are sent across a network, they may take different paths, and the data they carry might be fragmented into multiple pieces. The information in the header allows the receiving system to assemble these fragments back into the original message accurately, ensuring that the data is complete and correctly ordered. This aspect of data transmission is fundamental to network functionality and is why the correct answer highlights the importance of information for reassembling data across packets.

The other options focus on more specific or isolated elements. For instance, while the destination address is part of the header, it does not encompass the full range of what the header contains. Similarly, the type of data and encryption details are aspects that could be part of the header but do not capture the critical function of reassembly and routing across potentially fragmented or reordered packets in the broader context of networking.