I don't know the correct answer, but it seems C and D are not correct:
- "Do not record user events for product items that have not been imported yet."; i.e., import your product catalog first and then your user events.
- "Make sure that all required catalog information is included and correct. Do not use dummy or placeholder values."
https://cloud.google.com/retail/recommendations-ai/docs/upload-catalog#catalog_import_best_practices
I think the correct answer is B, because the "default optimization objective" for FBT is "revenue per order", whereas the "default optimization objective" for OYML is "click-through rate".
https://cloud.google.com/retail/recommendations-ai/docs/placements#fbt