It is like
<?php
use yiihelpersArrayHelper;
use backendmodelsStandard;
?>
<?= Html::activeDropDownList($model, 's_id',
ArrayHelper::map(Standard::find()->all(), 's_id', 'name')) ?>
ArrayHelper in Yii2 replaces the CHtml list data in Yii 1.1.[Please load array data from your controller]
EDIT
Load data from your controller.
Controller
$items = ArrayHelper::map(Standard::find()->all(), 's_id', 'name');
...
return $this->render('your_view',['model'=>$model, 'items'=>$items]);
In View
<?= Html::activeDropDownList($model, 's_id',$items) ?>
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