The correct setting to meet these requirements is to enable the option 'Sync Over Wi-Fi Only' within the photo synchronization settings of the smartphone. This ensures that photos are only synchronized to the work computer when the device is connected to a Wi-Fi network, thus avoiding the use of the limited mobile data plan. Meanwhile, disabling cellular data for photo synchronization would not be sufficient by itself because it would prevent photos from syncing entirely when on cellular networks, without providing a solution for syncing when Wi-Fi is available. Setting no restrictions or permitting background data usage could result in synchronization over cellular data, incurring charges and data usage against the employee's data plan.