What is the primary function of controller extensions in Salesforce?

The primary function of controller extensions in Salesforce is to enhance the capabilities of existing standard or custom controllers. By implementing a controller extension, developers can add additional properties and methods to a controller without altering the original controller's behavior. This allows for a more modular and reusable approach to building applications within Salesforce.

Controller extensions enable developers to customize the logic and functionality of visual elements, such as Visualforce pages, giving them the ability to extend standard operations or introduce new actions. This flexibility is particularly valuable when there's a need to maintain standard functionality while also implementing additional features that cater to specific business requirements.

In contrast, the other options do not accurately describe the purpose of controller extensions. Some options pertain to different aspects of the Salesforce platform, such as API communication or integration functions, while others incorrectly suggest limitations that are not characteristic of controller extensions. Thus, option A correctly outlines the role of controller extensions in extending the functionality of existing controllers in Salesforce applications.