You need to include columns which are required in your case its name,mobile and dob. So first do this
//MainEnquiry.aspx panel1
public object MainEnq_Stu_Name(EntityLayer.Entity en)
{
return dll.GetSingleValue("select name,mobile,dob from studentinfo where mobile=" + en.mainEnq_Stu_Mobile+"and dob is not null");
}
Then just else if conditions in your code. Hope you can go from here.