Use ACF array values in shortcode

$atts['form_id'] contains the form_id that was passed with the Shortcode. You are searching for this form_id among the data that is held in the forms field.

$forms is an array, and each item in the array is another array with your two keys: form_id and form_markup.

So, for each item in $forms, you want to check if that form_id matches the form_id requested in the Shortcode. if there is a match, return the form_markup from that item.

If no matches were found, return an error message.

$forms = get_field( 'forms', 'option' );
foreach ( $forms as $form ) {
    if( $atts['form_id'] == $form['form_id'] ){
        return $form['form_markup'];
    }
}

return 'form id ' . $atts['form_id'] . ' not found';