A. We can get any number of 1.
B.More strict .. means it is covering only one possibility of containing exactly two ones.eg. 0110 ,11 . But 0101 is also satisfying the condition which is not covered by option B.
C. It is covering all d possibility of containing exactly two one's . 101,11,0011,01010........
D. Any no. Of one we can take from here
So, answer is option C.