Economic profit is also known as abnormal or supernormal profit.

A is true. It is affected by changes in the structure of the economy such as changes in the rate of population growth, the rate of technological change. B is false. It is not a constant and tends to vary as the economy is subject to shocks. C is true. They may have different views of the growth rate, expected inflation etc. That's why it is not directly observable. The rate has to be estimated usually with a lot of uncertainty has to be estimated, usually with a lot of uncertainty.

When it comes to interest rate risk, not even government bonds are immune. In fact, a long term government bond's price will be a lot more volatile than a short term corporate bond's price. This is despite the fact that corporate bonds have a credit risk.