The class marks of a frequency distribution are 10, 15, 20, 25, …………… The class corresponding to the class mark 15 is :

1. 12.5-17.5

2. 10-20

3. 10.25-17

4. 14-16

The given class marks are 10, 15, 20, 25, ……..

The class width is the difference between any two consecutive class marks.

Class width = 15 - 10 = 5

If a is the class mark of a class interval of width h, then the lower and upper limits of the class interval are:

Lower limit = a - $\dfrac{h}{2}$ upper limit = a + $\dfrac{h}{2}$

So, a = 15 and h = 5

Class interval = $\Big(15 - \dfrac{5}{2}\Big) to \Big(15 + \dfrac{5}{2}\Big)$

= (15 - 2.5) to (15 + 2.5)

= 12.5 to 17.5

Hence, the class corresponding to the class mark 15 is 12.5-17.5.

Hence, option 1 is the correct option.