Question

For example, if you select the option with id="chocolate-yes" or id="vanilla-yes", the the option with id="icecreamcone-yes" will also be selected.

Anyone know of a Javascript that can do this?

Note 1: the option with id="sprinkles-yes" will not effect id="icecreamcone-yes".
Note 2: id names cannot be changed, nor can you add more properties to the elements.
Note 3: cannot use jQuery.

<ul id="icecream" style="list-style:none;line-height:30px;">
  <li>
    <select id="icecreamcone">
      <option value="addicecreamcone">Would you like an ice cream cone?</option>
      <option id="icecreamcone-yes" value="yes">Yes</option>
      <option id="icecreamcone-no" value="no">No thanks</option>
    </select>
  </li>
  <li>
    <select id="vanilla">
      <option value="addvanilla">Would you like to add vanilla ice cream?</option>
      <option id="vanilla-yes" value="yes">Yes</option>
      <option id="vanilla-no" value="no">No thanks</option>
    </select>
  </li>
  <li>
    <select id="chocolate">
      <option value="addchocolate">Would you like to add chocolate ice cream?</option>
      <option id="chocolate-yes" value="yes">Yes</option>
      <option id="chocolate-no" value="no">No thanks</option>
    </select>
  </li>
  <li>
    <select id="sprinkles">
      <option value="addsprinkles">Would you like to add sprinkles on top?</option>
      <option id="sprinkles-yes" value="yes">Yes</option>
      <option id="sprinkles-no" value="no">No thanks</option>
    </select>
  </li>
</ul>

Answer 1

Try this.

document.getElementById("chocolate").onchange = function(){
  if(document.getElementById("chocolate").options[1].selected == true) {
    document.getElementById("icecreamcone").options[1].selected = true
  }
}

When you select th select Chocolate IceCream to Yes, it will automatically set Icecrem Cone to Yes as well.