Answer:
A. Install host-based firewalls on all computers that have an email client installed
Step-by-step explanation:
Since the employee accidentally opened the email that contained malicious content and the computer on the company network got infected, it needs to install host-based firewalls on all computers that have an email client installed on the system. A host-based firewall is a type of firewall installed on individual client or server which controls the incoming and outgoing networking data and identifies whether to allow it or not. The host-based firewall can easily identify and block suspected exploits like viruses, worms or trojan horses.
By installing host-based firewalls on all computers that have an email client installed secures the entire network of computers.