Create and share a new folder for the assets in Content Builder.

The answer is A.
B & D are effectively the same thing. By moving an asset into a shared folder, you are, in fact, sharing an individual asset.

Now while B & D are technically feasible, the questions states there are "multiple" assets. Easiest way to move multiple assets, presuming they are in the same folder, is to share the folder

Answer B: Individually share each asset with the other business unit in Content Builder.