Updated

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  foos: ['hello', 'goodbye', 'maybe'],
  bar: 'maybe'
});

helpers.eq.js

import Ember from 'ember';

export function eq([a,b]/*, hash*/) {
  return a===b;
}

export default Ember.Helper.helper(eq);

styles.app.css

option[selected] {
  color: red;
  font-size: 2em;
  text-transform: uppercase;
  font-style: italic;
  font-family: monospace;
}

templates.application.hbs

Option for first select:
{{input value=bar}}<br>
<select>
  <option disabled>--Select One--</option>
  {{#each foos as |foo|}}
    <option selected={{eq foo bar}}>{{foo}}</option>
  {{/each}}
</select>

<select>
  <option disabled>--Select One--</option>
  {{#each foos as |foo index|}}
    <option selected={{if index false true}}>{{foo}}</option>
  {{/each}}
</select>

<select>
  <option disabled>--Select One--</option>
  {{#each foos as |foo|}}
    <option selected={{true}}>{{foo}}</option>
  {{/each}}
</select>

<select>
  <option disabled>--Select One--</option>
  {{#each foos as |foo|}}
    <option selected>{{foo}}</option>
  {{/each}}
</select>

<select>
  <option disabled>--Select One--</option>
  <option>hello</option>
  <option selected>goodbye</option>
  <option>maybe</option>
</select>

twiddle.json

{
  "version": "0.12.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "3.4.0",
    "ember-template-compiler": "3.4.0"
  },
  "addons": {
  }
}