A routing table is used when a destination IP address is not located on the current LAN segment. It consists of a list of station and network addresses and a corresponding gateway IP address further along to which a routing equipment should send packets that match that station or network address. A list of IP addresses and corresponding MAC addresses is an ARP table. A DNS is used to match host names and corresponding IP addresses. The last choice is a distracter. Source: STREBE, Matthew and PERKINS, Charles, Firewalls 24seven, Sybex 2000, Chapter 3: TCP/IP from a Security Viewpoint.

An audit trail is a set of records that collectively provide
documentary evidence of processing used to aid in tracing from
original transactions forward to related records and reports, and/or
backward from records and reports to their component source
transactions. Audit trails may be limited to specific events or may
encompass all of the activities on a system.
User audit trails can usually log:
All commands directly initiated by the user
All identification and authentication attempts
Files and resources accessed
It is most useful if options and parameters are also recorded from
commands. It is much more useful to know that a user tried to
delete a log file (e.g., to hide unauthorized actions) than to know the user merely issued the delete command, possibly for a personal data file.
*Answer "An audit trail is a device that permits simultaneous data processing of two or more security levels without risk of compromise." is a description of a multilevel devicE. A multilevel device is a device that is used in a manner that permits it to process data of two or more security levels simultaneously without risk of compromisE. To accomplish this, sensitivity labels are normally stored on the same physical medium and in the same form (i.e.,
machine-readable or human-readable) as the data being processed.
*Answer "An audit trail mediates all access to objects within the network by subjects within the network." refers to a network reference monitor, an access control concept that refers to an abstract machine that mediates all access to
objects within the network by subjects within the network.
* Answer "Audit trails are used to prevent access to sensitive systems by unauthorized personnel." is incorrect, because audit trails are detective, and the answer describes a preventative process, access control. Source:
NCSC-TG-001 A Guide to Understanding Audit in Trusted Systems
and DoD 5200.28-STD Department of Defense Trusted Computer
System Evaluation Criteria.

The answer: People. The other choices can be strengthened and counted on (For the most part) to remain consistent if properly protected. People are fallible and unpredictable. Most security intrusions are caused by employees. People get tired, careless, and greedy. They are not always reliable and may falter in following defined guidelines and best practices. Security professionals must install adequate prevention and detection controls and properly train all systems users Proper hiring and firing practices can eliminate certain risks. Security Awareness training is key to ensuring people are aware of risks and their responsibilities.
The following answers are incorrect:Software. Although software exploits are major threat and cause for concern, people are the weakest point in a security posture. Software can be removed, upgraded or patched to reduce risk. Communications. Although many attacks from inside and outside an organization use communication methods such as the network infrastructure, this is not the weakest point in a security posture. Communications can be monitored, devices installed or upgraded to reduce risk and react to attack attempts.
Hardware. Hardware components can be a weakness in a security posture, but they are not the weakest link of the choices provided. Access to hardware can be minimized by such measures as installing locks and monitoring access in and out of certain areas.
The following reference(s) were/was used to create this question: Shon Harris AIO v.3 P.19, 107-109 ISC2 OIG 2007, p.51-55