JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
   <li>
      <input name="alternativa" id="resp2" type="radio">
      <label for="resp2">Rhaego, Viserys e Drogo.</label>
   </li>
   <li>
      <input name="alternativa" id="resp3" type="radio">
      <label for="resp3">Drogon, Viserys e Aegon.</label>
   </li>
   <button onclick="avancar()">Teste</button>
</ul>

JavaScript

function avancar() {
    var alternativas = document.querySelectorAll("input[name=alternativa]:checked");

    if (alternativas.length < 1) {
        alert("Selecione ao menos uma resposta");
    }
    else {
    	//resto do codigo pra avancar pagina...
      window.location.href = "https://jsfiddle.net/about";
    }    
}