you can create one question with one option “Prefer not to disclose” on same page and that option will be multi select checkbox.
So based on checkbox click you can set the dropdown value as blank and using change event you can uncheck the checkbox.
generic code:
var checkbox = paste id of checkbox using inverted commas "#xyz"
var dropdown = paste id of select box using inverted commas "#abc"
jQuery(checkbox).click(function(){
if(jQuery(checkbox).is(":checked")){
jQuery(dropdown).val("");
}
});
jQuery(dropdown).change(function(){
jQuery(checkbox).prop("checked", false);
});
So both will become mutually exclusive to each other.