it should be C

I don't think the Streaming API is the correct option since its function is not for bulk data loading

I think the best answer is A for the following reasons :
A. Load Contact records together using the Streaming API via the Upsert operation.

This is a good approach. The Streaming API is designed for handling large volumes of data and can process records in near real-time. The Upsert operation is ideal when you have a mix of new and existing records, as it will insert new records and update existing ones based on the external ID field.
D. Load new records via the Insert operation and existing records via the Update operation.

This approach requires separating the data into two sets (new and existing records) before loading, which adds complexity and potential for error. It doesn't leverage the efficiency of the Upsert operation.