Question

I have a website in c#, in the first page I have some listbox, I need that
the last user’s choice goes to a label in other page, how can I do that?

In my code if
the user chooses a value a button is visible, and in the click event of that button
redirect to to another page, but I need that value in a label in the page2

if (ddlFunciones.SelectedValue.Equals("15"))
{
    lblAgregarNuevoServicio.Visible = true;
    //lblIdFuncion.Visible = true;
    lblDescripcion.Visible = true;
    //txtId_funcion.Visible = true;
    txtDescripcionFuncion.Visible = true;
    btnAgregarNuevaFuncion.Visible = true;
}


protected void RadioButtonList1_SelectedIndexChanged(object sender, EventArgs e)
{

}
protected void btnVerCargos_Click(object sender, EventArgs e)
{
    if (btnVerCargos.Enabled)
    {
        ListBoxCargo.Visible = true;
    }
    else
    {
        ListBoxCargo.Visible = false;
    }
}
protected void ListBoxCargo_SelectedIndexChanged(object sender, EventArgs e)
{

}

Answer 1

If this is in Asp.Net you can pass information between pages in a number of ways.
You can use the Session object

protected void ListBoxCargo_SelectedIndexChanged(object sender, EventArgs e)
{
   Session["MyVar"] = ListBoxCargo.SelectedValue;
}

and in your other page

object value;
if (Session["MyVar"] != null)
{
   value = Session["MyVar"]
}

OR

By passing them in the QueryString see Passing-variables-between-pages-using-QueryString
And using Request.QueryString["MyVar"]

and of course there are more, please explain what exactly are you trying to do…

Edit: Based on OPs comments:

In page1:

protected void Button1_Click(object sender, EventArgs e)
    {
        Session["Page1Value"] = ListBox3.SelectedItem.Text;
       //Response.Redirect("~/Page2.aspx");
    }

In Page2:

 protected void Page_Load(object sender, EventArgs e)
 {
    if (Session["Page1Value"] != null)
    {
       Label1.Text = Session["Page1Value"].ToString();
    }
 }