The correct option is B will buy
Answer : B
Explanation:
Quotation marks are used to mention the exact words spoken during a conversation.
So basically it can be understood as the written form of verbal communication.
And to talk about things that have not happened yet, we use the future tense.
Specifically we use the simple future tense to talk about things that will start and end in future.
Looking at the alternatives:
--will buy : "will" is used in sentences of future tense, and with it we use
the base / root form of the verb. Now here it is important to know that
we use "will" when we are making a decision at that particular moment or a prediction.
--am
going to buy : future tense; "going to" indicates the intention /
plan
of the speaker; and with that we use the base form of the verb. Please
note that we use "going to" when talking about a decision that was made
prior to the conversation.
--bought : past tense
--was buying : past continuous form of the verb
From the second sentence we get the idea that it was in that particular moment that the person got to know that the first person had passed the test, he/she decided to celebrate it with champagne. Thus it was a decision made at that very particular moment and was not pre-planned.
Thus grammatically the most suitable alternative is "will buy".
Therefore the correct complete sentence is:
"I passed my driving test!"
"That's great! I WILL BUY some champagne to celebrate!"