Type ft = null;
if (listBox选择A)
{
    ft = typeof(A窗体的类型);
}
else if (listBox选择B)
{
    ft = typeof(B窗体的类型);
}

foreach (Form item in Application.OpenForms)
{
    if (item.GetType() == ft)
    {
        item.Activate();
        break;
    }
}