Final answer:
The key data element on which the entire budget is based is the sales/revenue forecast.
Step-by-step explanation:
The key data element on which the entire budget is based is the sales/revenue forecast. This forecast predicts the amount of money a company or government expects to generate from sales during a specific period, usually a fiscal year. It is a crucial piece of information as it determines the expected income and helps in planning the allocation of funds for various expenses and investments.