<select id="countrySelect" onchange="changeBackgroundColor()">

  <option value="usa">USA</option>

  <option value="uk">UK</option>

  <option value="france">France</option>

</select>

​

<script>

  function changeBackgroundColor() {

    const selectedCountry = document.getElementById("countrySelect").value;

    if (selectedCountry === "usa") {

      document.body.style.backgroundColor = "red";

    } else if (selectedCountry === "uk") {

      document.body.style.backgroundColor = "blue";

    } else if (selectedCountry === "france") {

      document.body.style.backgroundColor = "white";

    }

  }

</script>