JSFiddle - React, Tailwind, and code Playground

HTML

<select id="sel">
    <option value="-1" >choisir...</option>
    <option value="un" >un</option>
    <option value="deux" >deux</option>
</select>
<label id="un"> un</label>
<label id="deux"> deux</label>

CSS

label {display: none;}

JavaScript

document.getElementById('sel').onchange=function(){
    switch(document.getElementById('sel').value) {
        case "-1": document.getElementById('un').style.display="none";
                 document.getElementById('deux').style.display="none"
                 break;
        
        case 'un': document.getElementById('un').style.display="block";
                 document.getElementById('deux').style.display="none"
                 break;    
       
        case "deux": document.getElementById('un').style.display="none";
                 document.getElementById('deux').style.display="block"
                 break;     
}
}