Ethernet is frame based network technology. See below a few definitions from RFC 1122:
SEGMENT A segment is the unit of end-to-end transmission in the TCP protocol. A segment consists of a TCP header followed by application data. A segment is transmitted by encapsulation inside an IP datagram.
PACKET A packet is the unit of data passed across the interface between the internet layer and the link layer. It includes an IP header and data. A packet may be a complete IP datagram or a fragment of an IP datagram.
FRAME A frame is the unit of transmission in a link layer protocol, and consists of a link-layer header followed by a packet.
The following answers are incorrect:
Ethernet segment. Is incorrect because Ethernet segment is a distractor, TCP segment would be the correct terminology. Ethernet is a frame based network technology,
Ethernet datagram. Is incorrect because Ethernet datagram is a distractor, IP datagram would be the correct terminology. Ethernet is a frame based network technology
Ethernet packet. Is incorrect because Ethernet packet is a distractor, a Packet is a group of information so would not be a "single unit". Ethernet is a frame based network technology.
Look at the diagrams below that were extracted from my Security+ Computer Based Tutorial. TCP/IP Data Structures

IMPORTANT NOTE:
The names used on the diagram above are from RFC 1122 which describe the DOD
Model.
Vendors and Books may use slightly different names or even number of layers.

TCP/IP Data Structure The following Reference(s) were used for this question:
Wikipedia http://en.wikipedia.org/wiki/Ethernet

Explanation/Reference:
A SYN attack occurs when an attacker floods the target system's small "in-process" queue with connection requests, but it does not respond when the target system replies to those requests. This causes the target system to "time out" while waiting for the proper response, which makes the system crash or become unusable. A buffer overflow attack occurs when a process receives much more data than expected. One common buffer overflow attack is the ping of death, where an attacker sends IP packets that exceed the maximum legal length (65535 octets). A smurf attack is an attack where the attacker spoofs the source IP address in an ICMP ECHO broadcast packet so it seems to have originated at the victim's system, in order to flood it with REPLY packets.
Source: KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten Domains of Computer Security, John Wiley & Sons, 2001, Chapter 3: Telecommunications and Network Security (page
76).

Explanation/Reference:
A bottom-up approach to testing begins testing of atomic units, such as programs or modules, and works upwards until a complete system testing has taken place. It allows errors in critical modules to be found early. A top-down approach allows for early detection of interface errors and raises confidence in the system, as programmers and users actually see a working system. White box testing is predicated on a close examination of procedural detail. Black box testing examines some aspect of the system with little regard for the internal logical structure of the software.
Source: Information Systems Audit and Control Association, Certified Information Systems Auditor 2002 review manual, Chapter 6: Business Application System Development, Acquisition, Implementation and Maintenance (page 300).
Top Down Testing: An approach to integration testing where the component at the top of the component hierarchy is tested first, with lower level components being simulated by stubs. Tested components are then used to test lower level components. The process is repeated until the lowest level components have been tested.
Bottom Up Testing: An approach to integration testing where the lowest level components are tested first, then used to facilitate the testing of higher level components. The process is repeated until the component at the top of the hierarchy is tested.
Black Box Testing: Testing based on an analysis of the specification of a piece of software without reference to its internal workings. The goal is to test how well the component conforms to the published requirements for the component.

DES is also known as a Symmetric Key or Secret Key algorithm.
DES is a Symmetric Key algorithm, meaning the same key is used for encryption and
decryption.
For the exam remember that:
DES key Sequence is 8 Bytes or 64 bits (8 x 8 = 64 bits)
DES has an Effective key length of only 56 Bits. 8 of the Bits are used for parity purpose
only.
DES has a total key length of 64 Bits.
The following answers are incorrect:
Two-key This is incorrect because DES uses the same key for encryption and decryption.
Asymmetric Key This is incorrect because DES is a Symmetric Key algorithm using the
same key for encryption and decryption and an Asymmetric Key algorithm uses both a
Public Key and a Private Key.
Public Key. This is incorrect because Public Key or algorithm Asymmetric Key does not use
the same key is used for encryption and decryption.
References used for this question: http://en.wikipedia.org/wiki/Data_Encryption_Standard

Explanation/Reference:
A communication channel that allows transfer of information in a manner that violates the system's security policy.
A covert channel is a way for an entity to receive information in an unauthorized manner. It is an information flow that is not controlled by a security mechanism. This type of information path was not developed for communication; thus, the system does not properly protect this path, because the developers never envisioned information being passed in this way.
Receiving information in this manner clearly violates the system's security policy. The channel to transfer this unauthorized data is the result of one of the following conditions:* Oversight in the development of the product
* Improper implementation of access controls
* Existence of a shared resource between the two entities
* Installation of a Trojan horse
The following answers are incorrect:
An undocumented backdoor that has been left by a programmer in an operating system is incorrect because it is not a means by which unauthorized transfer of information takes place. Such backdoor is usually referred to as a Maintenance Hook.
An open system port that should be closed is incorrect as it does not define a covert channel.
A trojan horse is incorrect because it is a program that looks like a useful program but when you install it it would include a bonus such as a Worm, Backdoor, or some other malware without the installer knowing about it.
Reference(s) used for this question:
Shon Harris AIO v3 , Chapter-5 : Security Models & Architecture
AIOv4 Security Architecture and Design (pages 343 - 344)
AIOv5 Security Architecture and Design (pages 345 - 346)