"7. mr gerry’s class has 19 students ms vernon’s class has 21 students and mr singh’s class has 23 students whose class has a composite number of students

Mr. Vernon's class (21 is divisible by 7 and three and thus is composite).

The major difference is that a histogram is only used to plot the frequency of score occurrences in a continuous data set that has been divided into classes, called bins.  bar charts, on the other hand, can be used for a great deal of other types ofvariables  including  ordinal  andnominal  data sets.