Which of the following is the proper syntax to specify a custom environment variable named...

GitHub Actions defines custom environment variables with the YAML env mapping. YAML mappings use a colon between each key and value, so MY_VARIABLE: my-value beneath env: is the correct syntax. The env key can appear at workflow, job, or step scope depending on how broadly the variable should be available. Options A, B, E, and F either use an unsupported key or the wrong assignment syntax. environment is associated with deployment environments when used in GitHub Actions workflow syntax; it is not the key for declaring ordinary environment variables. The vars context is used for configuration variables created through repository, organization, or environment settings, but var: is not the workflow syntax shown here. Therefore, option C correctly defines the requested custom environment variable.

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