The correct option is Lower cost for infrequent or bursty usage.

Azure Cosmos DB serverless billing charges for request units consumed and for storage, so an application that has intermittent or bursty traffic only pays for the capacity it actually uses. This makes the billing model more cost effective for workloads that are idle much of the time and that only need capacity during short spikes.

Because you do not provision and pay for fixed RU per second with serverless billing, you avoid paying for idle throughput and you do not need to manage RU provisioning for unpredictable traffic patterns. That operational simplicity and the consumption based cost model are the main reasons the Lower cost for infrequent or bursty usage option is correct.

Predictable monthly billing is incorrect because serverless is consumption based and not intended to provide a fixed monthly charge. Predictable monthly billing would describe a provisioned throughput or reserved capacity model instead.

Google Cloud Spanner is incorrect because it is a different database service from another cloud provider and it is not an advantage of an Azure Cosmos DB billing option.

Automatic expansion of stored data is incorrect in this context because automatic storage growth is a general platform behavior and it is not the key billing advantage of the serverless option. The serverless choice is primarily about how you are charged for throughput and not about special storage expansion capabilities.

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