The budget method that maintains a constant twelve-month planning horizon by adding a new month on the end as the current month is completed is called:

A continuous budget.  

Explanation:

A continuous budget -

It is the method of preparing a budget for future , where the alteration is done on the basis of current scenario , and can further be adjusted at the end of the time period , is referred to as a continuous budget .

This very process of budget making need to have current activity into consideration , as according to the current scenario the alterations are done to predict growth in the future.

Hence , from the information of the question,

The correct answer is a continuous budget .

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