According to the PMBOK® Guide, specifically within the Control Costs process, the Cost Performance Index (CPI) is a measure of the cost efficiency of budgeted resources. It is one of the most critical metrics in Earned Value Management (EVM).

The Formula: The CPI is calculated by dividing the Earned Value ($EV$) by the Actual Cost ($AC$).

$$CPI = \frac{EV}{AC}$$

Determining Factors: To calculate CPI, you must have:

Earned Value (EV): The measure of work performed expressed in terms of the budget authorized for that work.

Actual Cost (AC): The realized cost incurred for the work performed on an activity during a specific time period.

Significance: The CPI allows the project manager to determine if the project is over budget ($CPI < 1.0$) or under budget ($CPI > 1.0$) at a specific point in time.

Analysis of Other Options:

B. SPI (Schedule Performance Index): This is another performance metric ($EV / PV$). While it is part of the overall EVM suite, it is not used to calculate the CPI; rather, both are calculated using $EV$.

C. PV (Planned Value): PV is used to calculate the Schedule Variance (SV) and Schedule Performance Index (SPI). It represents the authorized budget assigned to scheduled work but does not factor into the cost efficiency (CPI) calculation.

D. ETC (Estimate to Complete): This is a forecasting metric that predicts the expected cost to finish all the remaining project work. While CPI is often used as a factor to calculate the Estimate at Completion (EAC), the ETC itself is not a factor used to determine the current CPI.