A ____ , like Adobe Reader, is software that can be added to increase the functionality of a browser.

The correct choice is a plug-in because this term specifically refers to software components that add specific capabilities or features to a larger software application, such as a web browser. Plug-ins enable browsers to display additional content types or provide functionality not natively supported by the browser.

In the case of Adobe Reader, it is a plug-in that allows a browser to view PDF documents directly. Users can install plug-ins to enhance their browsing experience with various functions, such as playing video or handling different file formats.

While the terms "add-on" and "extension" are also related to enhancing browser capabilities, they are often used interchangeably with "plug-in" but can have slightly different connotations depending on the context or the specific application. "Add-on" is a broader term that encompasses any software that adds features to a browser, and "extension" typically refers to user-modifiable features or tools that can be integrated into a browser's interface. Both of these terms can include plug-ins within their scope but do not capture the specific nature of plugins as enhancements that integrate directly into the browser’s functionality in a more technical manner.

A "program," on the other hand, is a more general term for software but does not specifically denote a component that integrates