Drupal form option populate from loop and also make a custom form options.

 

 $q = db_select('taxonomy_term_data','term')

 ->fields('term',array('tid','name'))

 ->condition('term.vid', 10);

 $q->join('field_data_field_tag_type','tag','term.tid = tag.entity_id');

 $results = $q->execute()->fetchAll();

 $options = array();

 foreach($results as $key => $data){

   $options[$data->tid] = $data->name;

 }

 $form['field_activity']["en"]["#options"] = $options;

Now Second Example:-

$loop = db_query('SELECT id, name FROM {table}')->fetchAllKeyed();

Hook form alter for select list

function myhook_form_alter(&$form, &$form_state, $form_id) {

  if($form_id == 'views_exposed_form'){

      $nodes = db_select('node', 'n')

        ->fields('n', array('title'))

        ->condition('type', 'news')

        ->orderBy('n.title', 'ASC')

        ->execute();

      $dropdown_array = array('' => t('-- Any --'));

      foreach ($nodes as $node) {

        $dropdown_array[$node->title] = $node->title;

      }

      // Alter form

      $form['title'] = array(     

         '#type' => 'select',

         '#default_value' => '1',        

         '#options' => $dropdown_array,

      ); 

   }//endif

}