XmlDocument xdoc = new XmlDocument();
            xdoc.LoadXml(xml);

            foreach (XmlNode node in xdoc.SelectNodes("/ArrayOfCategory/Category"))
            {
                Category question = new Category
                {
                    CategoryName = node.SelectSingleNode("/CategoryName").InnerText,
                    Id = Convert.ToInt32(node.SelectSingleNode("/Id").InnerText),
                };
                question.Print();
            }

<ArrayOfCategory xmlns="http://schemas.datacontract.org/2004/07/TeachERWcfService.Models" xmlns:i="http://www.w3.org/2001/XMLSchema-instance">
  <Category>
    <CategoryName>ddf</CategoryName>
    <Id>17</Id>
  </Category>
  <Category>
    <CategoryName>fff</CategoryName>
    <Id>18</Id>
  </Category>
  <Category>
    <CategoryName>fffg</CategoryName>
    <Id>19</Id>
  </Category>
  <Category>
    <CategoryName>fgf</CategoryName>
    <Id>20</Id>
  </Category>
  <Category>
    <CategoryName>gffvfg</CategoryName>
    <Id>21</Id>
  </Category>
</ArrayOfCategory>