For this you need to pass Dropdownlist value to XSL using parameter and based on param value which you have received in XSL apply xsl:sort statement. Roughly you need to do following
C# CODE
XslCompiledTransform tras = new XslCompiledTransform();
XsltArgumentList argList = new XsltArgumentList();
argList.AddParam("sortfield", "", "dd value");
tras.Transform("XMLfile", argList, sw);
XSL Header
<xsl:output method="html" indent="yes"/>
<xsl:param name="sortfield"></xsl:param>