public class Main
{
    public List<B> BList{ get; set; }
}
public class B
{
    public B()
    {
        ListA = new List<A>();
    }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Email { get; set; }
    public List<A> ListA { get; set; }
}
public class A
{
    public int Rating { get; set; }
    public int Weightage { get; set; }
}

Main main = new Main ();
var xEle = new XElement("Root", from x in main.BList
from te in x.A select new XElement("Child", new XElement("Weightage", te.Weightage), new XElement("FName", x.FirstName),new XElement("LName", x.Email) ));

<Root>
  <Child>
    <Weightage>10</Weightage>
    <FName>ABC</FName>
    <LName>ABC@gmail.com</LName>
  </Child>
  <Child>
    <Weightage>20</Weightage>
    <FName>ABC</FName>
    <LName>ABC@gmail.com</LName>
  </Child>
</Root>

<Root>
  <Child>
    <Weightage>10</Weightage>
    <Weightage>20</Weightage>
    <FName>ABC</FName>
    <LName>ABC@gmail.com</LName>
  </Child>
</Root>