The correct answer is option 'C': Checksum field.

Explanation:
The OSPF (Open Shortest Path First) protocol is a link-state routing protocol that is used to determine the best path for routing packets within an autonomous system (AS). OSPF uses a variety of fields in its header to transmit information between routers. One of these fields is the checksum field, which is used to detect errors in the packet.

The checksum field in the OSPF header is responsible for ensuring the integrity of the OSPF packet. It is used to detect any errors that may have occurred during transmission or processing of the packet. The checksum field contains a value that is calculated based on the contents of the packet, such as the OSPF header, OSPF packet length, and OSPF packet body.

When a router receives an OSPF packet, it recalculates the checksum based on the received packet's contents and compares it with the checksum value in the packet header. If the calculated checksum matches the value in the packet, it indicates that the packet has been received without any errors. However, if the calculated checksum does not match the value in the packet, it indicates that the packet has been corrupted or tampered with, and it is discarded by the receiving router.

By using the checksum field, OSPF ensures the reliability of packet transmission. It helps detect errors caused by transmission problems, such as bit errors, packet loss, or hardware malfunctions. If an error is detected, OSPF can take appropriate measures, such as requesting retransmission of the packet or selecting an alternate path for routing.

In conclusion, the checksum field in the OSPF header is used to detect errors in the OSPF packet. It plays a crucial role in ensuring the integrity and reliability of OSPF routing within a network.