Final answer:
A significant increase in network traffic indicates that a network is undergoing a DoS attack. Therefore, option c) a significant increase in network traffic is the condition that might indicate a network is undergoing a DoS attack.
Step-by-step explanation:
When a network is undergoing a DoS (Denial of Service) attack, a significant increase in network traffic is usually observed. This occurs because the attacker is flooding the network with a high volume of traffic, overwhelming its capacity and causing legitimate users to be unable to access the network resources. For example, an attacker might use a botnet to send a large number of requests to a website or a server, making it unable to respond to legitimate user requests. Therefore, option c) a significant increase in network traffic is the condition that might indicate a network is undergoing a DoS attack.