Script For Unchecking Dropdowns In Subscription Center

The contribution is definitely appreciated, but please don't do it this way!  It encourages poor coding practices (as does the post you based it on -- you can see my code revisions there as well).

You should use the Marketo Forms 2.0 API to change form values, not generic DOM methods, which will break in all but the most generic form.  You also can't use getElementById when there are multiple forms on a page, among other cases.

Do it like this, assuming your selects use the string "no" as opt-out.

MktoForms2.whenReady(function(form) {
   var masterField = "Unsubscribed",
      dependentFields = [
         "subscribe_resources__c",
         "subscribe_blog__c",
         "subscribe_webinars__c" 
         /* add more fields as needed */
      ],
      propagateMasterCheckedState = {
         true : "no", /* value of dependents when master checked */
         false : ""   /* value of dependents when master unchecked */
      }
   /* --- no need to edit below this line --- */
   var formEl = form.getFormElem()[0],
      masterEl = formEl.querySelector("[name='" + masterField + "']");
   masterEl.addEventListener("click", function(e) {
      var fieldsObj = {};
      dependentFields.forEach(function(field) {
         fieldsObj[field] = propagateMasterCheckedState[this.checked];
      },this);
      form.setValues(fieldsObj);
   });
});

Also, option 0 in your selects won't be "NULL" ("NULL" has special meaning on forms and is not added by default) it'll be the empty string.

Also, to help you learn the DRY principle in programming, here's how you refactor your original code toward less repetition and more maintainability:

document.getElementById("subscribe_resources__c").selectedIndex = 0;
document.getElementById("subscribe_blog__c").selectedIndex = 0;

simplifies to

document.getElementById("subscribe_resources__c").selectedIndex =
document.getElementById("subscribe_blog__c").selectedIndex = 0;

which simplifies to

[document.getElementById("subscribe_resources__c"),document.getElementById("subscribe_blog__c")] 
  .forEach(function(element){ 
    element.selectedIndex = 0; 
  });

which simplifies to

["subscribe_resources__c","subscribe_blog__c"]
  .forEach(function(id){ 
     document.getElementById(id).selectedIndex = 0; 
  });

which simplifies to (this is specific to how ID attributes work)

["subscribe_resources__c","subscribe_blog__c"]
  .forEach(function(id){ 
    window[id].selectedIndex = 0; 
  });

You can see how with each iteration, there's less repetition, less to type/mistype.

If you're certain you're in the global scope, you can even go further to:

[subscribe_resources__c,subscribe_blog__c]
  .forEach(function(id){ 
    id.selectedIndex = 0; 
  });

But I wouldn't do that last one unless you're really sure of the overall code architecture.

Thank you very much, @Sanford Whiteman​. I appreciate the help. I will edit my original post.