Hello Gmat_mission.
Let's take a look at your question.
(1) P = M+N where M and N are integers
This statement does tell us anything about P. We can have the following cases:
1. M=13, N=9, then P=M+N=22 and then P is divisible by 11.
2. M=13, N=2, then P=M+N=15 and then P is NOT divisible by 11.
Hence,
NOT SUFFICIENT.
(2) M is divisible by 11 and N is not divisible by 11
This statement only talks about M and N. Not about P. Hence,
NOT SUFFICIENT. .
Using both statements together
We have that P=M+N, where M and N are integer and M is divisible by 11 and N, is not.
Now, we have that $$\frac{M}{11}=\frac{M+N}{11}=\frac{M}{11}+\frac{N}{11}=integer\ +\ not\ integer\ =\ not\ integer.\ $$ This implies that M is not divisible by 11.
Hence this statement is
SUFFICIENT. .
The correct answer is the option
C.
I hope it helps.