GitLab CI/CD pipelines are configured using a YAML file named .gitlab-ci.yml located in the project's root directory. To integrate an external tool like Checkmarx, its predefined template is included in the main .gitlab-ci.yml file using the include: keyword. This allows the pipeline to execute the jobs defined in the Checkmarx template. The other filenames are not the standard convention recognized by GitLab for CI/CD configuration.

A. gitlab-cd.yml is not the standard filename for GitLab CI/CD configuration. GitLab specifically looks for .gitlab-ci.yml.

B. gitlab-ci/cd.yml is an incorrect filename and path. The configuration file must be in the project's root directory.

C. gitlab.yml is not the correct filename for defining CI/CD pipelines in GitLab.

1. GitLab Documentation. (n.d.). The .gitlab-ci.yml file. GitLab Docs. Retrieved from https://docs.gitlab.com/ee/ci/yaml/. In the introduction, it states, "To use GitLab CI/CD, you start with a .gitlab-ci.yml file in the root of your project."

2. GitLab Documentation. (n.d.). Keyword reference for the .gitlab-ci.yml file - include. GitLab Docs. Retrieved from https://docs.gitlab.com/ee/ci/yaml/index.html#include. This section details how to use the include keyword to incorporate external YAML files, including remote ones via URL.

3. Checkmarx Documentation. (2023). GitLab Integration. Retrieved from https://checkmarx.com/resource/documents/en/34965-6413-gitlab-integration.html. The integration guide specifies adding the Checkmarx template to the .gitlab-ci.yml file.