The correct answer is C because environment variables provide a secure method to pass credentials and secrets to MLflow Pyfunc models without hardcoding them in the code. This approach follows security best practices by keeping sensitive information separate from the application logic and preventing exposure in version control or logs. Option A (spark.conf.set) is unsuitable as it's for Spark configuration, not endpoint credential management. Option B (Databricks Feature Store API) is incorrect as it's for feature management, not credential passing. Option D (plain text) is a security anti-pattern that exposes credentials and should never be used.