Answer: A

A virtual private network (VPN) allows external partners to
securely participate in the extranet using public networks
as a transport or shared private networks. Because of its
low cost, using public networks (Internet) as a transport is
the principal method. VPNs rely on tunneling/encapsulation
techniques, which allow the Internet protocol (IP) to carry
a variety of different protocols (e.g., SNA, IPX, NETBEUI).
A client-server (choice B) does not address extending the
network to business partners (i.e., client-servers refers to
a group of computers within an organization connected by a
communications network where the client is the request
machine and the server is the supplying machine). Choice C
refers to remote users accessing a secured environment. It
is the means, not the method of providing access to a
network. A network service provider (choice D) may provide
services to a shared private network in providing Internet
services, but not extended to an organization's intranet.