Simple:
XDocument doc = new XDocument(
new XDeclaration("1.0", "utf-8", null),
new XElement("root",
new XElement("Textbox", YOUR_TEXTBOX_VALUE_HERE),
new XElement("combobox1", YOUR_COMBOBOX1_VALUE_HERE),
new XElement("combobox2", YOUR_COMBOBOX2_VALUE_HERE),
new XElement("combobox3", YOUR_COMBOBOX3_VALUE_HERE),
new XElement("table",
dt.AsEnumerble().Select(row => new XElement("rd",
new XElement("id", row["id"]),
new XElement("name", row["name"]),
new XElement("last", row["last"]),
new XElement("phone", row["phone"]),
new XElement("refF",
new XElement("adresse", row["adresse"]),
new XElement("citie", row["citie"]),
),
new XElement("age", row["age"]),
new XElement("mp", new XElement("degree", row["mp"])),
new XElement("dpa", row["dpa"])
))
)
)
);
doc.Save(FILENAME);