The requirement is to archive logs for seven years using Stackdriver (now Cloud Logging) while minimizing storage costs. Option D is correct because:

1. Creating a sink in Stackdriver allows exporting logs to Cloud Storage, which supports long-term archival.
2. Cloud Storage offers cost-effective storage classes like Archive or Coldline for infrequently accessed data, aligning with the seven-year retention need.
3. This approach uses native GCP tools without custom development, reducing operational overhead and ensuring reliability.

Option A is incorrect: Bypassing Stackdriver and sending logs directly to Cloud Storage misses Stackdriver's built-in export features and may complicate log management. Option B is incorrect: BigQuery is optimized for analytics, not archival, and incurs higher storage/query costs compared to Cloud Storage for long-term retention. Option C is incorrect: Pub/Sub is a messaging service, not permanent storage; it doesn't retain logs long-term and requires additional setup for archival.