答案：D

解析：

Choice D is correct. In a data set, standard deviation measures the amount of dispersion (variation) from the mean (average). A low standard deviation indicates that the data points tend to be close to the mean (low variability) and a high standard deviation indicates that the data points are more spread out relative to the mean (high variability).

Because the standard deviation of the heights of the students in Juan's math class (3.2 inches) is less than the standard deviation of the heights of the students in his English class (6.1 inches), it can be determined that the heights of the students in his math class have less variability than the heights of the students in his English class.