Answer: Yellow dominates portfolios Blue and Purple.
Step-by-step explanation:
Portfolio Yellow has a higher expected return than either portfolio Blue or Portfolio Purple which means that if we were evaluating the portfolios on return alone, Portfolio Yellow would dominate the other two.
However, we need to adjust for risk. The portfolio with the lowest standard deviation is the less riskier one of the three. That portfolio is Yellow which means that Yellow has both a higher expected return and a lower risk. It would therefore dominate the rest.