The correct answer is: A. required interest rate.
The real risk-free interest rate is the interest rate that would be earned on a risk-free investment if there were no inflation. The inflation premium is the additional amount of interest that is required to compensate for the expected rate of inflation. The required interest rate is the sum of the real risk-free interest rate and the inflation premium.
Option B, the quoted risk-free interest rate, is the interest rate that is actually quoted on a risk-free investment. However, this rate may not be the same as the real risk-free interest rate, because it may include a premium to compensate for inflation or other risks.
Option C, the liquidity risk-free interest rate, is the interest rate that is earned on a risk-free investment that is very liquid. This rate is lower than the quoted risk-free interest rate, because there is less risk of default on a liquid investment.
Option D, the premium risk-free interest rate, is the interest rate that is earned on a risk-free investment that is considered to be a safe haven. This rate is higher than the quoted risk-free interest rate, because there is less risk of default on a safe haven investment.