Two comments by experienced users (high reputations on other SE sites) on this popular question claim they don't have sufficient reputation to post an answer.
Since this is an open question I was under the impression that anyone could answer and posted a comment to that effect. If that is not true I would like to know so that I can delete my incorrect advice!
What is a “protected” or "highly active" question?
This question was protected which means that to answer the question you must have 10 rep. Unlike pretty much every other rep-gate (maybe there are some other exceptions), the association bonus does not count toward this limit.
One of the uses for "protect" is when there is a string of low-quality answers coming from people without experience in a particular site and the conventions and policies of that site.
It has been protected, therefore you need to earn 10 reputation on this site (having the association bonus is not enough) to post an answer.
There used to be a banner indicating this, but it's only shown if
That's why it's not immediately obvious to you.
In addition to the answers already given: The question is protected because it already has three deleted low quality answers. Hot network questions (and I think this one is one of those) attract a lot of "noise", answers from non-experienced users on a site which usually don't meet the criteria for a good answer.
