Option A meets all three requirements. The httpapi connection uses the Cisco NX-OS API instead of relying on an interactive CLI session. The serial: 1 directive restricts play execution to one managed switch at a time, ensuring that the switches are processed individually. The cisco.nxos.nxos_feature module is purpose-built for managing NX-OS features and operates idempotently: it enables OSPF when disabled but makes no unnecessary change when OSPF is already enabled. Option B uses network_cli, which does not satisfy the requirement to leverage the NX-OS API. Option C uses an obsolete local connection approach, while option D selects NETCONF and does not correctly implement the required one-device-at-a-time execution. The candidate file marks B, but the technically verified answer is A.