Communications security is the discipline of preventing unauthorized interceptors from accessing telecommunications in an intelligible form, while still delivering content to the intended recipients. In the United States Department of Defense culture, it is often referred to by the abbreviation COMSEC. The field includes cryptosecurity, transmission security, emission security, traffic-flow security and physical security of COMSEC equipment.

All of the other answers are incorrect answers:

Information security

Information security would be the overall program but communications security is the more specific and better answer. Information security means protecting information and information systems from unauthorized access, use, disclosure, disruption, modification, perusal, inspection, recording or destruction.

The terms information security, computer security and information assurance are frequently incorrectly used interchangeably. These fields are interrelated often and share the common goals of protecting the confidentiality, integrity and availability of information; however, there are some subtle differences between them.

These differences lie primarily in the approach to the subject, the methodologies used, and the areas of concentration. Information security is concerned with the confidentiality, integrity and availability of data regardless of the form the data may take: electronic, print, or other forms. Computer security can focus on ensuring the availability and correct operation of a computer system without concern for the information stored or processed by the computer.

Server security

While server security plays a part in the overall information security program, communications security is a better answer when talking about data over the network and preventing interception. See publication 800-123 listed in the reference below to learn more.

Client security

While client security plays a part in the overall information security program, communications security is a better answer. Securing the client would not prevent interception of data or capture of data over the network. Today people referred to this as endpoint security.

References:

http://csrc.nist.gov/publications/nistpubs/800-123/SP800-123.pdf

and

https://en.wikipedia.org/wiki/Information_security

and

https://en.wikipedia.org/wiki/Communications_security