When designing a row key for Cloud Bigtable, it's crucial to consider the access patterns to ensure efficient data retrieval. The MySQL table's move to Cloud Bigtable suggests that queries will frequently access data by Account_id and possibly by Event_timestamp. Therefore, combining Account_id with Event_timestamp in the row key (Account_id_Event_timestamp) ensures that all events for a specific account are stored contiguously and sorted by time, facilitating efficient range scans. This design supports common query patterns and optimizes performance in Cloud Bigtable. Options A and D are too simplistic and do not account for the time-based ordering, while option C reverses the order, which could lead to less efficient queries for account-specific data.