Which of the following is an example of a reconnaissance attack?

A reconnaissance attack is typically the initial phase of an attack where the attacker gathers information about the target to identify potential vulnerabilities. Scanning for open ports falls under this category as it involves probing a system to determine which ports are open and what services are running. By identifying open ports, an attacker can better understand the structure of the network and find ways to exploit it.

In contrast, phishing attempts primarily involve tricking individuals into revealing sensitive information, which is more about exploiting human behavior than gathering reconnaissance data about a system. Denial of Service attacks aim to disrupt the availability of a service rather than collect data. Ransomware infections are destructive attacks designed to encrypt data for ransom, which is far along the attack lifecycle and not associated with the reconnaissance phase. Thus, scanning for open ports stands out as the correct example of a reconnaissance attack.