the answer is b because 4/16 students are in detention. 25% means 1/4. if u break the group of 16 into four groups, there will be four students in each group (4+4+4+4=16) since four students are in detention, one full group will be in detention so 1/4. this is the best i can explain this but if you would like the mathematical proof work it is:
4 divided by 16 = 0.25 which turned into a percent is 25%