It's B Rank IIRC, but I wouldn't worry too much about the "Ranks" among DFs. Most high level DFs will all either allow for S/S+ Tier moves, or will at least allow the user to create custom S/S+ moves. The Letter Rankings mainly indicate how easy it is for the fruit to be overpowered.
For example, The Soru Soru no Mi is S Ranked just because it's very easy to make that fruit overpowered, while the Mochi Mochi is A Ranked because it's less easy to make the fruit as powerful comparatively, but it is still possible. Both the Mochi Mochi and the Soru Soru allow for S/S+ rank moves when mastered.