Amy has been asked to create a responsive navigation menu for a website. The navigation menu should have a drop-down feature that displays sub-menu items when clicked. Which CSS property can she use to create the drop-down feature?

Amy can use the CSS property "display" to create the drop-down feature for the navigation menu. Here are the steps she can follow:

1. First, she needs to structure her HTML to include a nested unordered list for the drop-down menu items. For example:

<ul>
  <li>Menu Item
    <ul class="dropdown">
      <li>Sub-menu Item 1</li>
      <li>Sub-menu Item 2</li>
    </ul>
  </li>
</ul>

2. Then, in her CSS, she should initially hide the drop-down menu by setting the "display" property to "none". For example:

.dropdown {
  display: none;
}

3. Finally, she can use the ":hover" pseudo-class to change the "display" property to "block" when the user hovers over the menu item. This will make the drop-down menu appear. For example:

li:hover .dropdown {
  display: block;
}

This way, when a user hovers over a menu item, the corresponding drop-down menu will be displayed.