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                <form name="form">
  <select name="item">
    <option value="">選択してください</option>
    <option value="item01">リンゴ</option>
    <option value="item02">ブドウ</option>
    <option value="item03">パイナップル</option>
<div class="item item01">リンゴが選択されました</div>
<div class="item item02">ブドウが選択されました</div>
<div class="item item03">パイナップルが選択されました</div>


                .item {
  display: none;
.item.selected {
  display: block;


                var item = document.querySelectorAll(".item");
var select = document.querySelector('[name="item"]');

// セレクトボックスを選択したら
select.addEventListener("change", function () {
  // selectedクラスを削除
  item.forEach(function (element) {
  // 選択したセレクトボックスのvalue値と同じクラス名の要素にselectedクラスを付与
  var selected = select.value;
  document.querySelector("." + selected).classList.add("selected");