JSFiddle - React, Tailwind, and code Playground

HTML

<form>
<select name="course" id="workshopcourseselect">
 <option value="choosecourse">Select Course</option>
 <option value="level4">BIFM Level 4</option>
 <option value="level5">BIFM Level 5</option>
 <option value="level6">BIFM Level 6</option>
    </select><br/>

<select name="module" id="workshopmoduleselect">
 <option value="choosecourse">Select Module</option>
 <option value="401" class="l4workshopmods">FM4.01 Understanding FM</option>
 <option value="402" class="l4workshopmods">FM4.02 FM Strategy</option>
 <option value="403" class="l4workshopmods">FM4.03 People Management in FM</option>
 <option value="404" class="l4workshopmods">FM4.04 Facilities Management Support Services Operations</option>
 <option value="405" class="l4workshopmods">FM4.05 Health &amp Safety</option>
 <option value="409" class="l4workshopmods">FM4.09 Performance Measurement in FM</option>
 <option value="415" class="l4workshopmods">FM4.15 Managing Customer Service in FM</option>
 <option value="417" class="l4workshopmods">FM4.17 Property, Fabric &amp Building Services Maintenance</option>
 <option value="419" class="l4workshopmods">FM4.19 Sustainability &amp Environmental Issues</option>
 <option value="421" class="l4workshopmods">FM4.21 Procurement &amp Contract Management for FMs</option>

 <option value="501" class="l5workshopmods">FM5.01 FM Development &amp Trends</option>
 <option value="502" class="l5workshopmods">FM5.02 Organisational &amp FM Strategy</option>
 <option value="503" class="l5workshopmods">FM5.03 Managing People in FM</option>
 <option value="504" class="l5workshopmods">FM5.04 Risk Management in FM</option>
 <option value="505" class="l5workshopmods">FM5.05 Financial Management in FM</option>
 <option value="511" class="l5workshopmods">FM5.11 Managing FM Projects</option>
 <option value="516" class="l5workshopmods">FM5.16 Propert &amp Asset Management in FM</option>
 <option value="521" class="l5workshopmods">FM5.21 Managing Procurement &amp Contracts in...

JavaScript

var l4 = $("#workshopmoduleselect .l4workshopmods").detach();
var l5 = $("#workshopmoduleselect .l5workshopmods").detach();
var l6 = $("#workshopmoduleselect .l6workshopmods").detach();
var def = $("#workshopmoduleselect option"); // everything else.  Don't detach.

$('#workshopcourseselect').change(function() {
    var level = $(this).val();
    var menu = $('#workshopmoduleselect');

    menu.empty();
    if (level == "level4")
        menu.append(l4);
    else if (level == "level5")
        menu.append(l5);
    else if (level == "level6")
        menu.append(l6);
    else
        menu.append(def);
});