I got C here but I am not sure

Let the number of girls be G and boys be 115-G.
I. Each girl=C cents
Each boy=C+50

GC+(115-G)*(C+50)=6900 Insufficient
II. Each girl=X cents
Each boy=2X cents
GX+(115-G)*2X=6900 ------(A) Insufficient
Combining,
C+50=2C
C=50=X
Substituting in eqn A,we can get G.
Hence,option C
Assumption:Total number of students are 115 only,not stated explicitly in question though.

I think the answer is B.
The answer would not change in DS question if we manipulate the data to make the analysis easier. Consider the following scenario:
we want to divide $9 dollars among the 3 students.
if we take condition 2 we know that total parts is 1+2 and the ratio is 1/2, hence girls would get 1/3 of the pot (9/3*1)=3 dollars, and therefore there is one girl, and boys would get 2 parts (9/3*2)= 6 dollars and there are 2 boys in the class.

condition 1 results in multiple solutions, and therefore is not enough.

Ans.C.Firdaus117 has adopted a very good approach.I couldn't have possibly solved that algebraically like Firdaus did.

if we take condition 2 we know that total parts is 1+2 and the ratio is 1/2, hence girls would get 1/3 of the pot (9/3*1)=3 dollars, and therefore there is one girl, and boys would get 2 parts (9/3*2)= 6 dollars and there are 2 boys in the class.

Condition 2 states that each boy gets twice that of each girl. However in your scenario above each girl and each boy are getting an equal amount - $3 each

arzanr wrote:

if we take condition 2 we know that total parts is 1+2 and the ratio is 1/2, hence girls would get 1/3 of the pot (9/3*1)=3 dollars, and therefore there is one girl, and boys would get 2 parts (9/3*2)= 6 dollars and there are 2 boys in the class.

Condition 2 states that each boy gets twice that of each girl. However in your scenario above each girl and each boy are getting an equal amount - $3 each

i realize my mistakes now... thanks.