A company is developing a product recommendation system that uses Amazon OpenSearch Service.

The correct answer is B because the scenario requires scalable approximate vector search, high recall, incremental updates, and complex filtering. Amazon OpenSearch Service supports k-NN vector search for recommendation use cases, and OpenSearch supports k-NN vector fields for high-dimensional vector search. The Lucene HNSW option is strongest here because OpenSearch documentation specifically states that Lucene supports k-NN searches using HNSW graphs and supports Lucene filters for k-NN searches. That directly matches the need for category and inventory filtering. Exact k-NN with Painless script scoring is accurate but too slow for 10 million vectors. IVF can be efficient but is less ideal for frequent incremental updates and complex filtering. Binary quantization reduces memory but sacrifices accuracy.