Final answer:
The answer that explains why bonds are known as fixed income securities is both b and c: investors know how much and when they will receive interest, as well as when the principal will be returned.
Step-by-step explanation:
Bonds are considered fixed income securities because they provide investors with a predictable income stream. This predictability comes from the fact that bonds typically pay interest payments at regular intervals and return the principal amount at the bond's maturity date. As such, the correct answer to why bonds are known as fixed income securities is D. Both b and c above are true.
An investor knows exactly how much interest they will receive and when. Moreover, upon the bond's maturity, they also know the exact amount of principal that will be paid back, assuming no default by the issuer. This contrasts with stocks, where dividends and returns are not guaranteed.