在下拉菜单选项中添加CSS矩形

<select id="shape-select">
  <option value="circle">Circle</option>
  <option value="rectangle">Rectangle</option>
  <option value="triangle">Triangle</option>
</select>

<select id="shape-select">
  <option value="circle">Circle</option>
  <option value="rectangle" class="rectangle-option">Rectangle</option>
  <option value="triangle">Triangle</option>
</select>

.rectangle-option {
  background-color: #f00;
  width: 100px;
  height: 50px;
}

<select id="shape-select">
  <option value="circle">Circle</option>
  <option value="rectangle" class="rectangle-option">Rectangle</option>
  <option value="triangle">Triangle</option>
</select>

<div id="shape-container">
  <div id="circle" class="shape">Circle</div>
  <div id="rectangle" class="shape">Rectangle</div>
  <div id="triangle" class="shape">Triangle</div>
</div>

<script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
<script>
$(document).ready(function() {
  $('#shape-select').change(function() {
    var selectedShape = $(this).val();
    $('.shape').hide();
    $('#' + selectedShape).show();
  });
});
</script>