<?php
/** @var array $params Current get params without c **/
$params = $_GET;
unset( $params[ 'c' ] );

/** @var string $url **/
$url = Yii::app()->controller->createUrl( Yii::app()->controller->id . '/' . Yii::app()->controller->action->id, $params );

/**
 * Create form with nedded c param included.
 */
echo TbHtml::beginFormTb(TbHtml::FORM_LAYOUT_INLINE, $url, 'GET', [ 'class' => 'plugin-submit-onchange' ]);
echo TbHtml::label('Number of items in list:', 'c');
echo TbHtml::dropDownList( 'c', $this->_c, $this->options );
echo TbHtml::endForm();
?>