Through the use of Public Key Infrastructure (PKI) the recipient's identity can be positively verified by the sender.
The sender of the message knows he is using a Public Key that belongs to a specific user. He can validate through the Certification Authority (CA) that a public key is in fact the valid public key of the receiver and the receiver is really who he claims to be. By using the public key of the recipient, only the recipient using the matching private key will be able to decrypt the message. When you wish to achieve confidentiality, you encrypt the message with the recipient public key.
If the sender would wish to prove to the recipient that he is really who he claims to be then the sender would apply a digital signature on the message before encrypting it with the public key of the receiver. This would provide Confidentiality and Authenticity of the message.
A PKI (Public Key Infrastructure) enables users of an insecure public network, such as the Internet, to securely and privately exchange data through the use of public key-pairs that are obtained and shared through a trusted authority, usually referred to as a Certificate Authority.
The PKI provides for digital certificates that can vouch for the identity of individuals or organizations, and for directory services that can store, and when necessary, revoke those
digital certificates. A PKI is the underlying technology that addresses the issue of trust in a
normally untrusted environment.
The following answers are incorrect:
The sender and recipient have reached a mutual agreement on the encryption key
exchange that they will use. Is incorrect because through the use of Public Key
Infrastructure (PKI), the parties do not have to have a mutual agreement. They have a
trusted 3rd party Certificate Authority to perform the verification of the sender.
The channels through which the information flows are secure. Is incorrect because the use
of Public Key Infrastructure (PKI) does nothing to secure the channels.
The sender of the message is the only other person with access to the recipient's private
key. Is incorrect because the sender does not have access to the recipient's private key
though Public Key Infrastructure (PKI).
Reference(s) used for this question:
OIG CBK Cryptography (pages 253 - 254)

Before the advent of classless addressing, one could tell the size of a network by the first few bits of an IP address. If the first bit was set to zero (the first byte being from 0 to 127), the address was a class A network. Values from 128 to 191 were used for class B networks whereas values between 192 and 223 were used for class C
networks. Class D, with values from 224 to 239 (the first three bits set to one and the fourth
to zero), was reserved for IP multicast.
Source: STREBE, Matthew and PERKINS, Charles, Firewalls 24seven, Sybex 2000,
Chapter 3: TCP/IP from a Security Viewpoint.

Explanation/Reference:
Security must be considered in information system design. Experience has shown it is very difficult to implement security measures properly and successfully after a system has been developed, so it should be integrated fully into the system life-cycle process. This includes establishing security policies, understanding the resulting security requirements, participating in the evaluation of security products, and finally in the engineering, design, implementation, and disposal of the system.
Source: STONEBURNER, Gary & al, National Institute of Standards and Technology (NIST), NIST Special Publication 800-27, Engineering Principles for Information Technology Security (A Baseline for Achieving Security), June 2001 (page 7).

Enticement deals with someone that is breaking the law. Entrapment encourages someone to commit a crime that the individual may or many have had no intention of committing. Enticement is not necessarily illegal but does raise ethical arguments and may not be admissible in court. Enticement lures someone toward some evidence (a honeypot would be a great example) after that individual has already committed a crime.
Entrapment is when you persuade someone to commit a crime when the person otherwise had no intention to commit a crime. Entrapment is committed by a law enforcement player where you get tricked into committing a crime for which you woud later on get arrested without knowing you rare committing such a scrime. It is illegal and unethical as well.
All other choices were not applicable and only detractors.
References:
TIPTON, Hal, (ISC)2, Introduction to the CISSP Exam presentation. and CISSP Study Guide (Conrad, Misenar, Feldman). Elsevier. 2010. p. 428 and http://www.dummies.com/how-to/content/security-certification-computer-forensics-andinci.html

Its primary function is to send messages between network devices regarding the health of the network. ARP matches an IP address to an Ethernet address. RARP matches and Ethernet address to an IP address. ICMP runs on top of IP. Source: KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten Domains of Computer Security, 2001, John Wiley & Sons, Page 87.