The GitHub REST API for Code Scanning provides specific endpoints to manage security alerts. The API includes an endpoint to list all code scanning alerts for a repository, which can be filtered by parameters such as the branch (ref) and state (open), thereby allowing a user to list all open alerts for the default branch. Additionally, there is a dedicated endpoint to retrieve the full details of a single alert using its unique alert number. These endpoints are fundamental for integrating code scanning results into external tools and custom workflows.

B. The API allows for updating an alert's state (e.g., dismissing it) but does not permit modification of the severity field, which is defined by the CodeQL query.

D. The Code Scanning API does not provide an endpoint to delete alerts. Alerts are part of the security history and can be dismissed or fixed, but not permanently deleted.

1. GitHub Docs, REST API, Code scanning, "List code scanning alerts for a repository": This official documentation page describes the GET /repos/{owner}/{repo}/code-scanning/alerts endpoint. The parameters table shows that you can filter results by ref (e.g., refs/heads/main for the default branch) and state (e.g., open), confirming option A.

2. GitHub Docs, REST API, Code scanning, "Get a code scanning alert": This page details the GET /repos/{owner}/{repo}/code-scanning/alerts/{alertnumber} endpoint, which directly corresponds to the action described in option C.

3. GitHub Docs, REST API, Code scanning, "Update a code scanning alert": This documentation for the PATCH /repos/{owner}/{repo}/code-scanning/alerts/{alertnumber} endpoint lists the updatable parameters. The list includes state and dismissedreason but explicitly omits severity, confirming that option B is incorrect. The documentation also lacks any mention of a "delete" operation, invalidating option D.