Binding an event handler to an HTML select.option

You can not use on click action in dropdown option. One solution is to use change on select element:

html

<input type="text" id="mail_address" />
<select onchange="warningaa(this);">
    <option value='google.com'>google.com</option>
    <option value='error'>error</option>
</select>

js

function warningaa(obj) {
    if(obj.value == "error") {
        alert('If you choose this option, you can not receive any infomation');
    }
}

fiddle

The option tag does not support the onclick event. Use the onchange event on the select instead.

HTML

<input type="text" id="mail_address"/>
<select id="selectbox" onchange="warning(this)">
  <option value='google.com'>google.com</option>
  <option value='warning'>Do not send me any kind of shit</option>
</select>

JS

function warning(obj) {
  if(obj.value == 'warning') {
    alert('If you choose this option, you can not receive any infomation');
  }
}