Final answer:
The risk measure that reflects unexpected performance is the standard deviation option (d), which indicates the variability of an investment's return from its expected outcome.
Step-by-step explanation:
The risk measure that reflects unexpected performance is the standard deviation. Standard deviation assesses the amount of variation or dispersion of a set of values. A low standard deviation indicates that the data points tend to be close to the mean (or expected value) of the set, while a high standard deviation indicates that the data points are spread out over a wider range.
This makes it a crucial tool for measuring the volatility or risk of an investment's return. Other measures like alpha and beta measure different aspects of performance and risk. Alpha represents the excess return on an investment relative to the return of a benchmark index. Beta measures the volatility of an investment relative to the overall market. The Sharpe ratio is used to understand the return on an investment compared to its risk.