Answer:
3/2
Step-by-step explanation: When selecting ratios for two variables (boys and girls) the two sides of the ratio must add up to be a factor of the total student count. The factors of 28 include 14, 7, 4, and 2. (1 + 1 = 2), (2 + 3 = 5), (3 + 4 = 7), and (1 + 3 = 4). 5 is the only nonfactor and cannot be the ratio of boys to girls, thus making 2 : 3 the correct answer.