Let the total no of books =t

Then the no of shelves in children's section = t/50

The no of shelves [smaller ones] =t/30

So basically the question is asking " What are the total no of books?"

1)

t/30 -t/50 =6 [ No of smaller shelves [t/30] are 6 + No of shelves in children's section [t/50]].You know children's because of the hypothetical were and the conditional would [ they weren't actually placed on them]

t[2/150]=6
t=450

We know total no of books .therefore we can find no of shelves.

2) Currently there are 9 shelves.

This too is sufficient because the question says " 50 picture books on each shelf in the children's section of a library",that is,no shelf is empty.

So total no of books = 50*9=450.

Since we know total,we can find the no of smaller shelves.

Hence sufficient

Answer should be D

[Remember: If there weren't the words 'on each shelf" ,B would have been insufficient,because we don't know on how many shelves,actually,there are books.]

Thanks. That helped!