Which DAX formula correctly calculates the previous month number?

The formula that correctly calculates the previous month number is the one that handles the transition from December to January correctly. This is accomplished through conditional logic that checks if the current month is January (1). If it is, the previous month should be December, which is represented by the number 12. For any other month, the formula simply subtracts 1 from the current month number, providing the correct previous month in those cases.

This approach ensures that the calculation accurately reflects the cyclical nature of months, addressing the edge case where the month rolls over from January back to December. By including this check in the logic, the formula ensures valid month representation throughout the entire year.

Other formulas fall short because they either don’t correctly account for the transition from January to December or do not engage with the month calculation effectively. For instance, subtracting 1 from the month does not consider when the current month is January, and similarly, simply using the MONTH function does not incorporate a mechanism to resolve the issue of cycling back to December. Thus, option B stands out as the comprehensive and accurate choice for calculating the previous month number.