The correct answer is A - Virtual private network (VPN).
According to WGU Cybersecurity Architecture and Engineering (KFO1 / D488), a VPN creates an encrypted tunnel over public or private networks, ensuring confidentiality, integrity, and authentication of data transmitted between geographically dispersed offices.
SIEM (B) manages security events, not secure communications. PPTP (C) is an outdated and insecure VPN protocol. NAC (D) enforces endpoint security but does not establish secure tunnels.
Reference Extract from Study Guide:
"A virtual private network (VPN) provides a secure, encrypted tunnel between geographically separated networks, enabling confidential communication over untrusted networks."
- WGU Cybersecurity Architecture and Engineering (KFO1 / D488), Secure Remote Access Technologies