The difference between the maximum and minimum observations is called

In statistics, the range of a dataset is the simplest measure of dispersion (spread): it is defined as the difference between the largest (maximum) observation and the smallest (minimum) observation, Range = Maximum − Minimum.

The question directly describes this definition — "the difference between the maximum and minimum observations" — so among the given options, this quantity is called the range.

The other three options name different statistical quantities, none of which is a difference between the dataset's extreme values:

• Frequency is the number of times a particular value or class occurs in the data — a count of occurrences, not a computed difference.

• Class size is the width of an interval used while grouping data into classes (the gap between a class's upper and lower limits) — a property of how the data is grouped, not of the whole dataset's extremes.

• Mean is the arithmetic average of all the observations (their sum divided by their count) — a measure of central tendency, not of spread.

So, among the options given, the term for the difference between the maximum and minimum observations is the range.