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For an iCloud backup to complete successfully, the device must be connected to a network, power source, and in a locked state. This ensures that the backup process can proceed without interruption, as an active device could potentially receive notifications, make calls, or open other apps, which might interfere with backing up data.

Being connected to a network is essential, as iCloud backups require internet access to upload data to Apple’s cloud servers. A power source is important because backups can consume a significant amount of battery power, and the process can take a long time, especially for larger sets of data. The device being locked is a requirement because when the device is locked, it's in a low-power state that minimizes interruptions, allowing for a smoother and more reliable backup process without user interaction.

This combination of requirements optimizes the backup performance and ensures that all data is securely backed up to the cloud. The other options do not cover all the necessary conditions, lacking either network or power connectivity, or assuming the device can be active and unlocked during the backup, which is not ideal for ensuring a complete and secure backup.