Cannot manage to work with a Select field in group

Did you set value for $var_options? Here is my test:

<?php
add_filter( 'rwmb_meta_boxes', 'your_prefix_register_meta_boxes' );
function your_prefix_register_meta_boxes( $meta_boxes ) {
    $prefix       = 'mbox_';
    $var_options  = array(
        'a' => 'First option',
        'b' => 'Second option',
    );
    $meta_boxes[] = array(
        'title' => esc_html__( 'Standard Fields', 'your-prefix' ),

        'fields' => array(
            array(
                'name'       => esc_html__( 'Depends of', 'pbc' ),
                'id'         => "{$prefix}depends",
                'type'       => 'group',
                'clone'      => true,
                'sort_clone' => true,
                'fields'     => array(
                    // SELECT BOX VARIATIONS
                    array(
                        'name'        => __( 'Variation', 'pbc' ),
                        'id'          => "{$prefix}depvar",
                        'type'        => 'select',
                        'options'     => $var_options,
                        'multiple'    => false,
                        'std'         => '',
                        'placeholder' => __( 'Select a Variation', 'pbc' ),
                    ),
                ),
            ), //array
        ),
    );

    return $meta_boxes;
}

And here is the result:

http://prntscr.com/bvycei