Final answer:
Tracing the packet flow back to the source during a DoS attack is difficult due to the complexity of the internet and attacker evasion techniques. It is generally attempted in severe cases where the attack has a substantial impact.
Step-by-step explanation:
Responding to a DoS (Denial of Service) attack typically involves technical and procedural steps, among which having your Internet Service Provider (ISP) trace the packet flow back to its source can be a part of the response plan. However, this is not as straightforward as it may seem. The difficulty of this task is largely due to the distributed nature of the internet and the sophisticated evasion techniques used by attackers. Moreover, IP spoofing can make it seem as though the attack comes from an innocent or non-existent IP address.
Given these complexities, the correct answer to the question would be 'c) Difficult; In severe cases'. It is difficult because of the nature of the internet and the anonymity that attackers can maintain, and it tends to be reserved for severe cases where the impact of the attack is significant enough to warrant the effort and resources required to pursue this course of action.