XML MSXML2.DOMDocument30 selectNodes

You will have to give XPATH correctly, I haven't tried using MSXML but it should be something like below.
/Site[Page[@id = 1]]. // node that you want to select and the filters within that node.
I tried using C# XMLDocument and SelectSingle node methods and it looks like below.

            XmlDocument doc = new XmlDocument();
            doc.Load(@"E:\Test.xml");
            XmlNode root = doc.DocumentElement;
            // Add the namespace.
            XmlNamespaceManager nsmgr = new XmlNamespaceManager(doc.NameTable);
            nsmgr.AddNamespace("st", "urn:sites-schema");
            XmlNode node = root.SelectSingleNode(
    "descendant::st:Site[st:Page[@id='1']]", nsmgr);
            Console.WriteLine(node.InnerXml);

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XML looks like below:

<?xml version='1.0'?>
<sites xmlns="urn:sites-schema">
  <Site some_attributes="xxx">
    <Page id="1"></Page>
    <Page id="2"></Page>
    <Style></Style>
  </Site>
</sites>

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You look at below reference for more help:
https://msdn.microsoft.com/en-us/library/cc189056%28v=vs.95%29.aspx
http://www.csharp-examples.net/xml-nodes-by-attribute-value/

Thanks for your reply,
the output I need is :

<Site some_attributes="xxx">
    <page id="1"></page>
</Site>

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You can use the below XPath:
   XmlNode node = root.SelectSingleNode(
    "descendant::st:Site/st:Page[@id='2']", nsmgr);
And get the outerXML
node.OuterXml
which shall give you
<Page id="2"></Page>
Then you may have to append the parent node's XML to get the desired value.
I shall check further if there is any other direct way to just the output that you desire (as per your last comment)

Thanks for your great efforts...

You are welcome!

Please take a look at this
https://www.experts-exchange.com/questions/28817899/Error-when-use-xsl-xslt-converter-xslt-to-convert-xsl-to-xslt.html