What can be used to define the instances to which Systems Manager will apply updates?

The correct choice refers to a patch group, which is a configuration used within AWS Systems Manager to specify which instances should receive specific software patch updates. By defining instances into patch groups, administrators can manage updates more effectively by targeting specific sets of instances that may share similar patches or update requirements.

This approach facilitates organized management, allowing updates to be applied selectively based on criteria such as operating system types, application requirements, or other factors relevant to the instances in the group. By using patch groups, systems administrators can ensure that their instances remain compliant and secure with the latest patches without having to manually apply updates to each instance individually.

In contrast, the other options serve different purposes within AWS Systems Manager. AppConfig is primarily intended for deploying application configuration settings rather than managing patch updates. A patch baseline is a set of rules for determining which patches to install and can be used in conjunction with patch groups. Explorer provides an overview and insights into the operational status of instances and resources, but it does not define instances for updates. Therefore, the specific role of patch groups as definers of instance targeting for updates makes it the most appropriate choice.