I am so confused can anyone help me understand which one is correct and why....

Beta is estimated as the slope of a regression line fit to pairs of periodic returns, (rx, ry), where: rx is the return for a market index such as the S&P 500 Index. rx is the return for the stock being analyzed for example, IBM s return if we are estimating IBM s beta. the slope measures the average return for the market portfolio for each percentage change in the value of the security of interest. ry is the return for the market index such as the S&P 500 Index.

A. rx is the return for a market index such as the S&P 500 Index.

B. rx is the return for the stock being analyzed for example, IBM s return if we are estimating IBM s beta.

C. the slope measures the average return for the market portfolio for each percentage change in the value of the security of interest.

D. ry is the return for the market index such as the S&P 500 Index.