3. An attack in which the site is not capable of answering valid request.Select one:a.Smurfingb.Denial of servicec.E-mail bombingd.Ping storm

The correct answer is b. Denial of Service.

A Denial of Service (DoS) attack is a malicious attempt to make a server or a network resource unavailable to users, usually by temporarily interrupting or suspending the services of a host connected to the Internet. In a DoS attack, the attacker usually sends excessive messages asking the network or server to authenticate requests that have invalid return addresses.

Here's a step by step breakdown:

1. The attacker begins by choosing the target server or network.
2. The attacker then sends a flood of traffic or data to the target server/network. This can be done through various methods, such as sending more traffic than the server can handle or exploiting system vulnerabilities.
3. The server is overwhelmed by the flood of traffic or data and can no longer process valid requests from legitimate users.
4. As a result, the server becomes unavailable, denying service to legitimate users.

This is why it's called a "Denial of Service" attack. The other options (Smurfing, E-mail bombing, Ping storm) are specific types of DoS attacks or related concepts, but the most general term for an attack that makes a site incapable of answering valid requests is a Denial of Service attack.