It's D) The hard part is the backend data storage, not the frontend.
People's intuition of how user data storage and retrieval works at Google scale doesn't align with the reality. Cost of storage (immediate and secondary), effect on creation of new permissions, privacy protection, user security (can this new option somehow be used to leak userdata or trick users into doing something they didn't want to do?), and (if they're only rolling this out in Europe) region gating all have to be accounted for.
People's intuition of how user data storage and retrieval works at Google scale doesn't align with the reality. Cost of storage (immediate and secondary), effect on creation of new permissions, privacy protection, user security (can this new option somehow be used to leak userdata or trick users into doing something they didn't want to do?), and (if they're only rolling this out in Europe) region gating all have to be accounted for.