The correct answer is: A. security market line.
The security market line (SML) is a line that represents the relationship between the expected return of a security and its beta. The SML is a graphical representation of the CAPM, which states that the expected return of a security is equal to the risk-free rate plus a risk premium that is proportional to the beta of the security.
The beta of a security is a measure of its volatility relative to the market. A security with a beta of 1 has the same volatility as the market, a security with a beta of 2 is twice as volatile as the market, and a security with a beta of 0.5 is half as volatile as the market.
The SML is upward sloping because investors require a higher expected return for securities that are more volatile. The SML is also flatter for securities that are less volatile. This is because investors are willing to accept a lower expected return for securities that are less risky.
The SML is a useful tool for investors because it can be used to estimate the expected return of a security. The SML can also be used to compare the risk and return of different securities.
The other options are incorrect because they do not accurately represent the relationship between risk and required return.
Option B, required return line, is a more general term that can refer to any line that represents the relationship between risk and return. However, the SML is a specific type of required return line that is based on the CAPM.
Option C, market risk line, is a term that is sometimes used to refer to the SML. However, the SML is more accurately described as a line that represents the relationship between risk and required return, while the market risk line is a line that represents the relationship between beta and expected return.
Option D, risky return line, is a term that is not commonly used in finance.