The rate at which the net present value (NPV) is equal to zero is:

✅Explanation:
The internal rate of return (IRR) is a financial metric used to evaluate the profitability of potential investments. It is the discount rate that makes the net present value (NPV) of all cash flows from a project equal to zero. In other words, it is the rate at which the present value of expected cash inflows equals the present value of expected cash outflows.

📌Other Options:
-(a) Benefit-cost ratio (BCR): This is a ratio that compares the present value of benefits to the present value of costs for a project. A BCR greater than 1 indicates a potentially profitable project.
-(b) Net present value (NPV): This is the difference between the present value of cash inflows and the present value of cash outflows over a period of time. A positive NPV indicates a potentially profitable project.
-(c) Payback period: This is the length of time it takes for a project to recoup its initial investment. A shorter payback period is generally preferred.