My answers may not be 100% technically correct but:
(a) the xmlns namespace is built-in to XML processors so you don't have to define them yourself. I'm not sure if it has a URI associated with it but I have sometimes seen http://www.w3.org/2000/xmlns/
specified, I think this also applies to part b of your question.
It doesn't count as an attribute though, it's a way of defining namespaces in your document so the answer to (c) is neither.
For question 2 XML schemas normally use the prefix xs or xsd and this is bound to the http://www.w3.org/2001/XMLSchema
URI. However attributes only usually reside in a namespace is they have a prefix, they don't assume the default namespace as elements can. So the attributes you quote don't have an associated namespace URI.