Solution:
We have \(2, 5, 15, 25, 20, 12, 8, 7, 6, 16, 21,17, 30, 32, 23, 40, 51, 15, 2, 9, 57, 19, 25\)
The data lie in a class interval \(x-y\) include all the data between \(x\) and \(y\) including \(x\).
Now, observation \(20,21,23,\) lie in the class interval \(20-25\).
So, frequency of class interval \(20-25\) is \(3.\)
Hence, the option \((C)\) is correct.