Answer:
I'm going to get some from the shop.
Explanation:
Use "be going to" when you either intend to do something in the future (though it may not actually happen after all) OR when you have previously made a plan to do something in the future and you are talking now about it.
Use "will" when you intend to do something in the future (though it may not actually happen after all) OR when you decide something at the moment of speaking.
The "B" response MUST be " be going to" because the speaker says "I know." That means he already (in the past) was thinking about a future action.
