Which of the following elements can be added to the _liferay-look-and-feel.xml file to modify the theme?

The choice of application decorators is accurate because the _liferay-look-and-feel.xml file is specifically designed to customize certain aspects of a Liferay theme, including the structure and layout adjustments that can be achieved through decorators. Application decorators allow developers to wrap the entire portal's output with a specific HTML structure, which can be vital for implementing distinct visual presentations or overriding default output with custom elements. This capability to extend the functionality by changing the layout and styling of sections of the site is pivotal in theme development within Liferay.

Other choices like custom CSS, custom JavaScript, and HTML metadata are indeed important elements in theme creation and manipulation within Liferay, but they are not specifically configured in the _liferay-look-and-feel.xml file. Custom CSS and JavaScript are generally included in the theme's stylesheet or script files instead and can be linked from other configuration files or folders. Meanwhile, HTML metadata is usually specified in the head section of HTML files rather than centralized in the _liferay-look-and-feel.xml. Hence, the focus on application decorators as the correct answer highlights the particular purpose and functionality bestowed by the _liferay-look-and-feel.xml configuration in customizing themes.