Final answer:
The reason Apex tests were run during a manual deployment to UAT is because the 'Run all local tests' checkbox is enabled in the UAT environment settings. The correct answer is (c).
Step-by-step explanation:
The correct answer is (c) The Run all local tests checkbox is enabled in the UAT environment. When Debbie is doing a manual deployment to move her Apex class to UAT, if the 'Run all local tests' checkbox is enabled in the UAT environment, Apex tests will be run automatically. This can be checked and modified by the Salesforce administrator in the UAT environment settings. Therefore, Debbie should check the UAT environment settings and disable the checkbox if she doesn't want the Apex tests to run during the deployment.