How to make a drop down list in yii2?

It seems you've found your answer already but since you mentioned the active form I'll contribute with one more, even if it differs only ever so slightly.

<?php
    $form = ActiveForm::begin();

    echo $form->field($model, 'attribute')
        ->dropDownList(
            $items,           // Flat array ('id'=>'label')
            ['prompt'=>'']    // options
        );

    ActiveForm::end();
?>

It is like

<?php
use yii\helpers\ArrayHelper;
use backend\models\Standard;
?>

<?= 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) ?>