(B) 17.5 – 22.5
Explanation:
According to the question,
Difference between class marks = 5
The classes for these class marks are as follows:
2.5 – 7.5
7.5 – 12.5
12.5 – 17.5
17.5 – 22.5
22.5 – 27.5
Hence, the class corresponding to class mark 20 = 17.5 – 22.5
Hence, option (B) is the correct answer.