Development is the correct option.

Software maintenance is generally independent of the Development phase because maintenance describes corrective, adaptive, perfective, and preventive activities that occur after a system has been delivered. Maintenance work typically focuses on keeping the system operational and improving it in production and it does not require repeating the entire original development lifecycle for each change.

Acceptance is incorrect because acceptance activities remain relevant to maintenance when changes are delivered to users. Customer acceptance or acceptance testing is often required for maintenance releases that affect functionality or service level agreements.

Testing is incorrect because testing is an essential part of maintenance. Patches and enhancements must be verified by testing to prevent regressions and to validate that fixes address the reported issues.

Operations is incorrect because maintenance is closely tied to the operations phase. Maintenance activities are performed while the system is in production and they interact directly with operational concerns such as deployment, monitoring, and incident handling.

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