To ensure real-time ingestion of user activity data into BigQuery, the most efficient and scalable solution involves using Cloud Pub/Sub in conjunction with a Dataflow streaming job. Cloud Pub/Sub acts as a real-time messaging service to capture the data from the mobile applications, while Dataflow processes and streams this data into BigQuery. This approach is scalable, cost-effective, and capable of handling high volumes of data with low latency. Option B is more complex and resource-intensive for this specific need. Option C is not feasible because Cloud Pub/Sub alone cannot process or store data; it requires a consumer like Dataflow to handle the data. Option D lacks a direct data source from mobile apps without a message queue like Cloud Pub/Sub, making it less optimal for real-time ingestion.