Expected Returns on a stock is defined as the "average return on a risky asset expected in the future...", which begs the questions of what is meant by "risky asset". And, realized returns may be more or less than the expected return. What measures the risk of a stock when we know that the return on a stock can vary greatly? Which of the following information is needed about a risky stock under study to determine its risk?

a$.$ The historic & present distributions of the stock's prices.

b$.$ The mean of the observed stock prices.

c$.$ The sum of the squared variances of the stock's prices.

d$.+1-$ Square root of the variances $=$ standard deviation of the mean.

e$.$ All of the above.