In physics in general, and quantum mechanics in particular, there is great interest, and substantial disagreement about fundamental, foundational issues. These questions are at least sometimes deep and subtle. A tag such as foundations-of-physics or fundamental-question or just "foundations" might be appropriate for questions related to these issues. What is the best tag for this type of question?
But I am more concerned about a second, somewhat different sort of basic question. In particular, I am concerned about very basic questions concerning basic concepts and definitions, or elementary procedures. You would think that these questions would be easily answered by googling or consulting an elementary text book. But I find it is amazing how many very elementary questions I cannot answer by those means, and so I turn to Physics SE. These questions are not homework, and should not be treated as homework (i.e. hints instead of clear answers). Often the purpose of the question is to clear up a misconception. So the homework tag is very inapropriate for this type of question. Perhaps basic-question would be an appropriate tag or even simpler-than-homework or some other such tag. I don't know how many others do, but I certainly feel the need for a tag of this type. Comments and suggestions?
PS I just read the following, based on the automated suggestion, and have not changed my mind.
There is already tag foundations (and it's not meta by any means). Just it is not used that much (yet?).
The problem you are addressing is very real, as most of simple questions can be asked on totally different grounds, from a newbie to the most knowledgable person on that topic. See e.g. Negative probabilities in quantum physics where I got some newbie answers.
basic-question is a meta tag plus I guess that most of questions would need it, so it is a bad idea to introduce it (even though my approach to meta tags is very liberal).
I don't think it's necessary to have a tag to identify these types of questions. These types of tags (which give general metadata about a question instead of characterizing its subjects) are called "meta tags," and there is plenty of discussion on Meta Stack Overflow (and even some on this meta site) about why they should be avoided. (In fact I personally think we could do without the homework tag, but the community seems to like it and it's too well-entrenched to get rid of now.)
The questions themselves, of course, are fine; I just don't think they need to be tagged in the way you're talking about.