As always, anything could happen but working with info we have at this moment, Sauro has a D. Everyone in the family supposedly has a D. After Foxy arc, Luffy`s life was saved because Ao Kiji chose to spare him because of something his grandfather did which put Ao Kiji in
This is a bit off-topic, but I have never seen Aokiji's comment in this way. I think the proper quote is below (from stephen's translation):
Aokiji: It would be so easy to crush you here
and end your life… but I have a debt to pay.
Let's call the stakes even... now that you've taken care of Crocodile for me.
Now, to me, it seems that it's Luffy he owes a favor to, for taking Crocodile out. All I recall him saying about the grandfather was that he had:
Aokiji: You see… I owed your grandfather a "favor" once, years ago...
The reason I'm here is just to take a look at Nico Robin... and you.
And I think...
You ought to die now.
Now, maybe it's just me, but this doesn't strike me as the reason he spared Luffy. I think it's something Aokiji did years ago for Luffy's granddad.
Letting Robin escape from Ohara, perhaps? Which would just reinforce the Sauro as Granddad theory…