<script>
var options = document.getElementsByTagName('option');
for (var i = 0; i < options.length; i++) {
options[i].innerHTML = (i + 1) + '. ' + options[i].innerHTML;
}
</script>

<select>
<option value="volvo">Volvo</option>
<option value="saab">Saab</option>
<option value="mercedes">Mercedes</option>
<option value="audi">Audi</option>
</select>
<script>
var options = document.getElementsByTagName('option');
for (var i = 0; i < options.length; i++) {
options[i].innerHTML = (i + 1) + '. ' + options[i].innerHTML;
}
</script>