I think it is E.
The only inference from the stem is that the total mean of the two classes is the average of the two means. Unless we have some kind of relationship about the marks, it is tough to determine individual means from either (1) or (2) or combined and thus the total deviation.
SD = sqrt(sum of squares of the mean deviations/total number of samples)
What we need to find out is the mean of all the students of both the classes and then compute the SD. By the very nature of the above formula(sqrt) and the statistics(two same groups may have different means but their mean deviations sum might be the same), we cannot say that SD = SD1+SD2 or anything of that kind.
Mrs. Hopkins has two math classes, each with the same number of students. What was the standard deviation of all her math students' scores on an identical test she gave both classes?
1) The standard deviation of the test scores in her first math class was 2 points.
2) The standard deviation of the test scores in her second math class was 2 points.
Thnx & Rgds,