A distributed denial of service attack can BEST be described as:

A.
Invalid characters being entered into a field in a database application.

B.
Users attempting to input random or invalid data into fields within a web browser application.

C.
Multiple computers attacking a single target in an organized attempt to deplete its resources.

D.
Multiple attackers attempting to gain elevated privileges on a target system.

Explanation:
A Distributed Denial of Service (DDoS) attack is an attack from several different computers targeting a single computer.
One common method of attack involves saturating the target machine with external communications requests, so much so that it cannot respond to legitimate
traffic, or responds so slowly as to be rendered essentially unavailable. Such attacks usually lead to a server overload.
A distributed denial-of-service (DDoS) attack occurs when multiple systems flood the bandwidth or resources of a targeted system, usually one or more web
servers. Such an attack is often the result of multiple compromised systems (for example a botnet) flooding the targeted system with traffic. When a server is
overloaded with connections, new connections can no longer be accepted. The major advantages to an attacker of using a distributed denial-of-service attack are
that multiple machines can generate more attack traffic than one machine, multiple attack machines are harder to turn off than one attack machine, and that the
behavior of each attack machine can be stealthier, making it harder to track and shut down. These attacker advantages cause challenges for defense
mechanisms. For example, merely purchasing more incoming bandwidth than the current volume of the attack might not help, because the attacker might be able
to simply add more attack machines. This after all will end up completely crashing a website for periods of time.
Malware can carry DDoS attack mechanisms; one of the better-known examples of this was MyDoom. Its DoS mechanism was triggered on a specific date and
time. This type of DDoS involved hardcoding the target IP address prior to release of the malware and no further interaction was necessary to launch the attack.
Incorrect Answers:
A: Invalid characters being entered into a field in a database application does not describe a DDoS attack. Therefore, this answer is incorrect.
B: Users attempting to input random or invalid data into fields within a web browser application does not describe a DDoS attack. Therefore, this answer is
incorrect.
D: Multiple attackers attempting to gain elevated privileges on a target system does not describe a DDoS attack. In “distributed denial-of-service”, distributed
means multiple computers, not multiple attackers. Therefore, this answer is incorrect.

http://en.wikipedia.org/wiki/Denial-of-service_attack