JSFiddle - React, Tailwind, and code Playground

HTML

<div class="Value">
    <select name="variation[1]" class="VariationSelect" style="width: 180px;">
        <option value="">Choose a Ring Size</option>
        <option value="6">6</option>
        <option value="7">7</option>
        <option value="21">Custom</option>
    </select>
</div>
<div id="cont_21" style="margin-left:20px; float:left; display:none"> 
    <div class="Label" style="margin-top:5px; ">
        Custom Size:&nbsp;&nbsp;
    </div>
</div>

JavaScript

$(".VariationSelect").change(function() {
    if ($(this).val() == '21') {
        $("#cont_21").show();
    } else {
        $("#cont_21").hide();
    }
});