The correct options are
A Total number of people in 0-20 age group is 10.
C Maximum number of people are from age group of 21-30 years.
Length of the class interval is varying in this case.
In such case, frequency per unit class has to be calculated and histogram has to be constructed with respective heights. For convenience, least class length is taken as unit class length.
Modified length=Frequency of the classLength of that class×Least Class length
In our case,
Modified length = Length of red bar = 5.
Length of class interval = 20-0 = 20.
Least class length = 40-30 = 10.
Frequency = 10.
Hence option A is correct and B is incorrect.
The class of 21-30 age group has the highest frequency.
Hence option C is correct and D is incorrect.