a- is correct. 

Let's see:

Some is common in affirmative clauses(+).

Any is common in questions and negatives(-).

for example:
I want some rice. (NOT I want any rice.)
Have you got any rice? (More natural than ‘Have you got some rice?’)

Some can also be used in questions(?), when we expect or want to encourage people to answer ‘yes’.

For example, some is common in offers and requests.
Would you like some more cheese? (The speaker is encouraging the listener to say ‘yes’)
Could I have some meat, please? (The speaker expects the listener to say ‘yes’.)

Any in affirmative clauses(+)
Any can be used in affirmative clauses after negative words such as never, hardly, without, little etc.
There is hardly any milk left.
There is little point in arguing any more.
You never give me any help.

 some (+/?); any (-/?/+with hardly,little, never)

I hope that will help.