How can you reduce latency when adding multiple expenses as separate entities in Cloud Datastore?

Utilizing a batch operation to add multiple entities in one request is an effective way to reduce latency in Cloud Datastore. When you batch multiple write operations, you minimize the overhead associated with each individual network round trip that would occur if you were to insert each entity separately. This consolidated approach allows you to send a single request to the Datastore service rather than multiple requests, thus drastically cutting down communication times.

Batching is particularly efficient because it optimally uses the available resources and reduces the overall transaction time, enabling faster processing and acknowledgments from the server. This method is ideal for scenarios where multiple entities need to be added, creating a more efficient and streamlined insertion process.

In contrast, the other options suggest methods that may not directly address latency issues related to network overhead or may even introduce additional complexity. For example, avoiding built-in indexes might improve insert times slightly but can lead to other performance issues in querying later. Automatically generated keys with numeric IDs may simplify key management but don't significantly impact latency for inserts. Similarly, composite indexes are more relevant to optimizing queries rather than enhancing insert performance. Therefore, option D stands out as the most effective strategy for reducing latency when adding multiple expenses as separate entities in Cloud Datastore.