The correct answer is D. effective implementation.
The last step in developing an operating budget is to implement it effectively. This means ensuring that the budget is used to track and control expenses, and that it is used to make informed decisions about future spending.
Option A, implementing income, is not a step in developing an operating budget. Income is a factor that is considered when developing a budget, but it is not a step in the process.
Option B, implementing decision, is also not a step in developing an operating budget. Decisions about how to spend money are made throughout the budget process, but there is no specific step called “implementing decision.”
Option C, efficient implementation, is a possible goal of implementing an operating budget, but it is not the last step in the process. The last step is to ensure that the budget is implemented effectively, which means more than just being efficient.
Therefore, the correct answer is D. effective implementation.