The correct option is Azure Pipelines variable marked as secret.

The Azure Pipelines variable marked as secret option is correct because secret variables live inside the pipeline and can be referenced directly by deployment tasks without adding an external retrieval step. These variables are masked in logs and can be injected into steps that call the external monitoring service so the confidential API key is passed securely with minimal changes to the existing pipeline definitions.

Variable group in Azure DevOps linked to Key Vault is not the best choice when the goal is the least modifications because linking a variable group to Key Vault requires configuring the variable group, granting access to the Key Vault, and ensuring the pipeline references the group. Those changes add configuration and possible permission steps compared with a native pipeline secret.

Secret stored in Azure Key Vault is also not the least disruptive option because pulling a secret from Key Vault at runtime requires creating a service connection or adding a Key Vault task and managing access permissions. That introduces extra steps and pipeline changes beyond using an existing pipeline secret.

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