That question is closed as a duplicate of a different question. There isn't any need to "mark it as answered" in any way, for a couple of reasons:
Users who aren't logged in (and, for example, find that question through search results) will automatically be redirected to the duplicate target. Open an Incognito tab and click the link in your meta question to try it; you won't see the unanswered question!
Closed questions don't show up in the unanswered tab (note that the link there is for the unanswered tab of remote-access), which the question is tagged with. It also won't count as unanswered in our site stats.
In our system, a question is considered 'answered' in the unanswered questions tab if it has at least one answer with score > 0 or accepted. So essentially, in direct answer to your question, no, there's no way to manually mark it as solved, but there's no need to do that anyway!
By the way: welcome to the site!