Assuming this is part of a larger XML document, since the fragment you've presented is not valid XML on its own, you need to select the element using the namespace-qualified name.
For example, using LINQ-to-XML:
const string input = "<root><id root=\"44da1b5a-c56a-46a4-bcb0-fde7784c8409\" xmlns=\"urn:hl7-org:v3\"/></root>";
XDocument document = XDocument.Parse(input);
XNamespace ns = "urn:hl7-org:v3";
XElement id = document.Descendants(ns + "id").First();
string root = (string)id.Attribute("root");
Overview - LINQ to XML | Microsoft Docs[
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