A link rot is the process of a hyperlink on a website being made permanently unavailable.
It can happen for many reasons. Including content being removed for not being legal anymore or for a change in the technology of the platform.
It can even be because of a change of domains, a change in the code, or even because the link expired.
In any of those situations, the reason for a Link Rot is most likely:
C. The sites publisher has done a poor job maintaining the site
That because all the mentioned problems can be solved if you're taking care of your website.