<select name='property_types' id='property_types' class='postform' onchange="zz()">


        <option class="level-0" value="Digital screen">Digital Screen</option>

        <option class="level-0" value="Lamp poster">Lamp poster</option>

        <option class="level-0" value="Bus stand">Bus Stand</option>

        <option class="level-0" value="Indoor shopping mall">Indoor shopping  
mall</option>

        <option class="level-0" value="Green belts">Green belts</option>

        <option class="level-0" value="showUnhide">others..</option>

    </select>

    <input type="text" id="textareaTobeShown" style="display:none">


<script>
function zz()
{
var aa=document.getElementById("property_types");
if(aa.value=="showUnhide")
{
document.getElementById('textareaTobeShown').style.display= '';
}
else
{
document.getElementById('textareaTobeShown').style.display= 'none';

}

}
</script>