toggle.cs

public class ToggleCallback : MonoBehaviour
{
  public TextMeshProUGUI label;
  public List<Toggle> toggles;

  void Start()
  {
    string s = toggles
                  .Select(t => t)
                  .Where(t => t.isOn == true)
                  .First()
                    .name.Substring(8, 1);
    setLabel(s);
  }

  public void setLabel(string s)
  {
    if (toggles.All(t => !t.isOn))
        s = "";

    label.text = "Toggle selected:" + s;
  }
}