[Gurthang]

I would actually go with 'D'. It might have been that they were forbidden to retrieve them.

Since Feanor didn't give the Silmarils to the Valar, the Valar had no right to them. Manwe might have considered them stolen property if they had been brought back and given to him. It is very possible that they, Aule and Ulmo, would have known where the Silmarils were, but knowing that they would have known I think they must have been told not to go get them. If that is not the case, I would have to go with option 'C': they didn't know/couldn't find them.

But it could also be that the Valar no longer had use for the Silmarils. The Two Trees had been dead for 10,000(?) years. I think that in the Silmarillion it says that if the Silmarils had been brought back quick enough, the Trees could have been saved. With so much time passed, the Trees were likely dead beyond recalling. And since the Trees could not be brought back using the Silmarils, the Valar had no more need of them.