try this..:)
DataTable dt = new DataTable();
using (SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["ConnectionName"].ConnectionString))
{
using (SqlCommand cmd = new SqlCommand("select Mark from TableName where Name="+ ComboBox.SelectedValue, con))
{
con.Open();
SqlDataAdapter da = new SqlDataAdapter(cmd);
da.Fill(dt);
lblMark.Text = Convert.ToString(dt.Rows["Mark"]);
con.Close();
con.Dispose();
cmd.Dispose();
}
}