(c) classes preceding the median class
The cumulative frequency of a class is obtained by adding the frequencies of all the classes preceding the given class.
Source: Chapter 13, Section 13.5 Summary (Point 3)
The textbook's Summary explicitly states: "The cumulative frequency of a class is the frequency obtained by adding the frequencies of all the classes preceding the given class." In the median formula, cf refers to the cumulative frequency of the class just before the median class. Students often confuse this with "up to the median class" — but the median class's own frequency is not included in cf.