Let the frequency of the first group be a, then the frequencies of the fourth, second and third groups are a+0. 1, a+0.2 and a+0.3 respectively.
Because the sum of all frequencies is 1,
So a+a+0.1+a+0.2+a+0.3 =1,
The solution is a=0. 1,
The frequency of the first group is 10,
All n =10 0.1=100.
So choose C.