If a patch baseline is not explicitly defined in Patch Manager, it is attached to the default patch group. This default group applies a preconfigured baseline with AWS-recommended patches, ensuring basic compliance for instances without custom baselines. The WGU Cloud Deployment and Operations Study Guide (Section 5.2, Patch Manager) states, 'If no custom patch baseline is defined, instances are associated with the default patch group, which uses AWS-provided baseline settings for automatic patch approval.' Options A, C, and D are not valid attachments for patch baselines.