A Slow HTTP POST attack is a type of denial-of-service (DoS) attack that exploits the way web servers handle HTTP requests. The attacker sends a legitimate HTTP POST header to the web server, specifying a large amount of data to be sent in the request body. However, the attacker then sends the data very slowly, keeping the connection open and occupying the server’s resources. The attacker can launch multiple such connections, exceeding the server’s capacity to handle concurrent requests and preventing legitimate users from accessing the web server.

The attack duration D is given by the formula D = a * b, where a is the number of connections and b is the hold-up time per connection. The attacker intends to maximize D by manipulating a and b. The server can manage m connections per second, but any connections exceeding m will overwhelm the system. Therefore, the scenario that is most likely to result in the longest duration of server unavailability is the one where a > m and b is the largest. Among the four options, this is the case for option B, where a = 100, m = 90, and b = 15. In this scenario, D = 100 * 15 = 1500 seconds, which is the longest among the four options. Option A has a larger b, but a < m, so the server can handle the connections without being overwhelmed. Option C has a > m, but a smaller b, so the attack duration is shorter. Option D has a > m, but a smaller b and a smaller difference between a and m, so the attack duration is also shorter.