One of your clients has opened a branch office in another state. Both the main office and the new branch office have fast, reliable Internet access. You have been asked to connect the branch office with the main office with a secure connection that uses their existing network infrastructure. What do you recommend?

Site-site VPN

Explanation:

In Computer Networking, a site to site Virtual Private Network (VPN) is used to securely connect two networks using the existing Internet connection available at both network infrastructure.

Basically, a site-site VPN extends the main office' network to the branch office thereby making computer resources available to employees over there.