Final answer:
The correct answer is C. firewall. An extranet requires stringent security measures, and a firewall serves as the primary defense system, managing access and protecting the network against unauthorized intrusion.
Step-by-step explanation:
A company implementing an extranet setup must place significant emphasis on security to protect sensitive data accessible through the extranet. The best option from the choices provided for ensuring a secure extranet environment is a C. firewall. A firewall acts as a security barrier, controlling the incoming and outgoing network traffic based on an applied set of security rules. This is crucial for preventing unauthorized access to the network and is a fundamental component of a company's network infrastructure when establishing an extranet, which connects the company's internal network with external entities such as business partners, suppliers, or customers.