please quickly

According to the mean-variance criterion, which one of the following investments dominates all others?

Select one:

a. E(r) = 0.15; Variance = 0.20

b. E(r) = 0.15; Variance = 0.18

c. E(r) = 0.05; Variance = 0.25

d. E(r) = 0.25; Variance = 0.15