'A' is used for future fulfilment of the present, for the future result of a present cause. In predictions, we use be going to when we have outside evidence for what we say. I have the symptoms that betoken my sickness in the future.
'B' is used when you are intending to do something very soon.
'C' is used to talk about what is going to happen in the future, especially things that you are certain about or things that are planned.
'D' is used for official plans and arrangements, fate, precondition, and order.
I would go for A, but I think C is not incorrect either.
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