Alter form to have single radio button term reference checked by default I have a taxonomy reference field and in the node creation form there will be only one value allowed and I guess I use a radio button and I want it to be enabled by default. 

I thought I can do it with form_alter but on inspection in krumo I see the default values are set to the concrete term id. 

If I would want to set that id (which I do not know) it could look like: 

 form['field_abteilung'][LANGUAGE_NONE]['#default_value'] = 12345;


For various users and their roles there will be other another allowed option in this form and I just want to do sth like 

 form['field_abteilung'][LANGUAGE_NONE].... My-single-radio-button = checked;


Update

I saw I can find out the single option by looking into 

$form['field_sektion'][LANGUAGE_NONE]['#options'][2] 


I tried it but get an error about missing index 'field_sektion' which confuses me. 

update 2
It takes some time, I maybe should have mentioned that the field in the end should be the access control field of https://drupal.org/project/workbench_access , that module also alters the form; I disabled the field's access function first and now need to rebuild all
the way I did it now, is following - First I tried to use a hook provided by workbench access itself which supposed to be for that use case, look here
https://drupal.org/node/2019473

But it would not work for me, so I did : 
this results in a select box with only one possibility. Not the best UX style maybe and later I will try to hide the element, maybe with ds module.

function workbench_access_crud_by_state_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form_id != 'hardware_antrag_node_form') {
    return;
  }
  //sdpm($form);
  $option_arr = $form['field_tu'][LANGUAGE_NONE]['#options'];
  // remove 'none'-option
  array_shift($option_arr);

  // look for field for workbench_access and replace options
  $wb_access_field = $form['workbench_access_fields']['#value'][0];
  $form[$wb_access_field][LANGUAGE_NONE]['#options'] = $option_arr;
}