The median class for the given distribution is:

Class Interval	Frequency
0 - 10	2
10 - 20	4
20 - 30	3
30 - 40	5

1. 0 - 10

2. 10 - 20

3. 20 - 30

4. 30 - 40

The given class intervals are already in ascending order. We construct the cumulative frequency table as under :

Here, Cumulative frequency = 14, which is even.

By formula,

$\text{Median }= \dfrac{\dfrac{n}{2}\text{th observation} + \Big(\dfrac{n}{2} + 1\Big)\text{th observation}}{2}\\[1em] = \dfrac{\dfrac{14}{2}\text{th observation} + \Big(\dfrac{14}{2} + 1\Big)\text{th observation}}{2}\\[1em] = \dfrac{7\text{th observation} + (7 + 1)\text{th observation}}{2}\\[1em] = \dfrac{7\text{th observation} + 8\text{th observation}}{2}\\[1em]$

All observations from 7th to 8th are equal, each lies in the interval 20 - 30.

So, median class = 20 - 30

Hence, option 3 is the correct option.