When Calculating the NPV,what Discount Rate Should We Use

When calculating the NPV,what discount rate should we use?

A) A discount rate equal to the investment return that could be earned on nonmarketable securities with similar risk characteristics.
B) A discount rate equal to the investment return that could be earned on marketable securities with similar risk characteristics.
C) A discount rate equal to the investment return that could be earned on low-risk marketable securities.
D) A discount rate equal to the investment return that could be earned on high-risk nonmarketable securities.
E) None of the above.