The height of a rectangle in a histogram shows the
Frequency of the class
A histogram is a graphical representation of a grouped frequency distribution with continuous classes. It is an area diagram and can be defined as a set of rectangles with bases along with the intervals between class boundaries and with areas proportional to frequencies in the corresponding classes. A histogram is a diagram involving rectangles whose area is proportional to the frequency of a variable and width is equal to the class interval.
Hence, option d is correct answer.