<form id=categorySelection>
    <label>
        <select>
            <option value="" disabled selected>SELECT CAT</option>
            <option data-idToShow=test1>Einkaufen & Dienstleistungen</option>
        </select>
    </label>
    <label id=test1 class=displayNone>
        <select>
            <option value="" disabled selected>PLEASE SELECT</option>
            <option data-idToShow=catSelect1>CAT 1</option>
            <option>Webseiten</option>
        </select>
    </label>
    <label id=catSelect1 class=displayNone>
        <select>
            <option value="" disabled selected>PLEASE SELECT</option>
            <option data-idToShow=catSelect2>CAT 1</option>
            <option>CAT X</option>
            <option>CAT Y</option>
        </select>
    </label>
    <label id=catSelect2 class=displayNone>
        <select>
            <option value="" disabled selected>PLEASE SELECT</option>
            <option>CAT a</option>
            <option>CAT b</option>
            <option>CAT c</option>
            <option data-idToShow=catSelect3>CAT D</option>
            <option>CAT</option>
            <option>CAT</option>
            <option data-idToShow=catSelect4>CAT</option>
        </select>
    </label>
    <label id=catSelect4 class=displayNone>
        <select>
            <option value="" disabled selected>PLEASE SELECT</option>
            <option>CAT</option>
            <option data-idToShow=catSelect5>CAT</option>
        </select>
    </label>
    <label id=catSelect5 class=displayNone>
        <select>
            <option value="" disabled selected>PLEASE SELECT</option>
            <option>CAT</option>
            <option>CAT</option>
            <option>CAT</option>
            <option>CAT</option>
            <option>CAT</option>
            <option>CAT</option>
        </select>
    </label>
    <label id=catSelectXXX class=displayNone>
        <select>
            <option value="" disabled selected>PLEASE SELECT</option>
            <option>CAT</option>
            <option>CAT</option>
            <option>CAT</option>
        </select>
    </label>
    <label id=catSelectXXX class=displayNone>
        <select>
            <option value="" disabled selected>PLEASE SELECT</option>
            <option>CAT</option>
            <option>CAT</option>
            <option>CAT</option>
        </select>
    </label>
    <label id=catSelectXXX class=displayNone>
        <select>
            <option value="" disabled selected>Bitte wähle die Art des Kurses</option>
            <option>CAT</option>
            <option>CAT</option>
            <option>CAT</option>
        </select>
    </label>
</form>
<script>

    (function categorySelection() {

        var f = document.getElementById( "categorySelection" );

        f.onchange = function() {

                hideAllVisibleSubElements();
                var els = document.querySelectorAll('form#categorySelection > label > select > option');
                for(var i = 0; i < els.length; i++) {
                        var e = els[i];
                        var idToShow = e.getAttribute('data-idToShow');

                        if( e.selected === true && idToShow !== null ) {
                            console.log("HTML: " + e.innerHTML + " selected: " + e.selected + " idToShow: " + idToShow);
                            showId(idToShow);
                        }
                }
        }
    }

    function hideAllVisibleSubElements( ) {

        var els = document.querySelectorAll('#categorySelection > label:nth-of-type(1n+2) > select');
        /*
        for(var i = 0; i < els.length; i++) {
            var e = els[i];
                e.selectedIndex = 0;
        }
        */
        var els = document.querySelectorAll('#categorySelection > label:nth-of-type(1n+2)');

        for(var i = 0; i < els.length; i++) {
            var e = els[i];
            if( !e.hasClass('displayNone') ){
                e.addClass('displayNone');
            }
        }
    }

    function showId( id ) {
        var e = document.getElementById( id );
        if( e.hasClass('displayNone') ){
            e.removeClass('displayNone');
        }
    }

    categorySelection();
</script>