how to get the post id in the option tag

$post->id needs to be $post->ID

You can also use get_the_ID()

Since your calling setup_postdata() you need to call wp_reset_postdata() after your foreach loop.

You also need to echo the value of $options somewhere.

foreach( $myposts as $post ) :  setup_postdata($post);
                $options .= '<option>';
                $options .= '<option value="'.$post->id.'">';
                $options .=  get_the_title();
                $options .= '</option>';
                    echo $options;
        endforeach;