Final answer:
A firewall is the correct option for preventing access to specific websites to protect children from accidental access. It can be configured to block certain sites.
Step-by-step explanation:
To prevent access to specific websites and ensure online safety for their children, users can deploy a number of solutions. However, the most applicable option from the ones given is D. A firewall. A firewall can be configured to block access to specific websites or IP addresses. In addition to hardware solutions, there are also software firewalls and parental control applications that can be particularly user-friendly for this purpose. They allow for the creation of blacklists, which are lists of undesired websites that the firewall will automatically block. Another alternative not listed but worth mentioning is utilizing the router's built-in security features, where you can also often set parental controls and create access schedules.