<h3>Ice Cream </h3>
 <form method="post" name="ice_cream" id="ice_cream" action="https://cs12.net/form/submit.php">
   <div>Would you like ice cream?
     <label>
       <input type="radio" name="want" id="ic_yes" value="yes"/>
Yes     </label>
     <label>
       <input type="radio" name="want" id="ic_no" value="no"/>
No     </label>
   </div>
   <fieldset id="icecream_options3">
     <legend>Ice Cream Options     </legend>
     <p>Cup or a Cone?
     </p>
     <label>
       <input type="radio" id="container_cup" name="container" value="cup"/>
Cup     </label>
     <label>
       <input type="radio" id="container_cone" name="container" value="cone"/>
Cone     </label>
     <p>Pick your toppings:
     </p>
     <label>
       <input type="checkbox" name="toppings" id="toppings_wc" value="whipcream"/>
Whipped cream     </label>
     <label>
       <input type="checkbox" name="toppings" id="toppings_jimmies" value="jimmies"/>
Jimmies     </label>
     <label>
       <input type="checkbox" name="toppings" id="toppings_sprinkles" value="sprinkles"/>
Sprinkles     </label>
     <label>
       <input type="checkbox" name="toppings" id="toppings_nuts" value="nuts"/>
Nuts     </label>
     <label>
       <input type="checkbox" name="toppings" id="toppings_cherry" value="cherry"/>
Cherry     </label>   </fieldset>
   <p>
     <input type="submit"/>

   </p>
 </form>  

In style element (<style>) within head element:

body {font-family: Calibri,Tahoma,Verdana,Helvetica,sans-serif;}
label { display: block;}

input[type=submit] {display: block; margin-top: 1rem;}

#icecream_options3 {
  background-color: rgb(238, 238, 238);
  margin-left: 1em;
}

In script element within head element (<script>):

"use strict";

let inputsWant;
console.log("hello from ice_cream.js");

window.addEventListener("DOMContentLoaded", function () {
  /* here's where JS goes for when DOM is loaded */

  /* select ice cream options and hide */
  document.getElementById('icecream_options3').style.display = 'none';

  /* add click listener to yes and no input choices
     use "querySelectorAll", which lets us use CSS selectors!
     querySelectorAll returns a list of things that we can
     loop through via 'for ( item of list ) { }'
     */
  inputsWant = document.querySelectorAll("form input[name=want]");
  console.log(inputsWant);
  for (let input of inputsWant) {
    console.log(input);
    input.addEventListener("click", iceCreamOptionsDisplay);
  }
});

function iceCreamOptionsDisplay() {
  let icoEl, inputYes;
  console.log("in iceCreamOptionsDisplay");
  icoEl = document.getElementById("icecream_options3");
  console.log(icoEl);
  inputYes = document.getElementById("ic_yes");
  if (inputYes.checked == true) {
    icoEl.style.display = "block";
  } else {
    icoEl.style.display = "none";
  }
}