The correct option is
A Little
'Little' and 'a little' are used with single uncountable nouns.
'Few' and 'a few' are used with plural countable nouns.
Options C and D: 'Little' emphasizes a small amount of something.
'A little' removes the emphasis: 'a' removes the emphasis from 'small amount'.
In the sentence, the emphasis is laid on him opening any lock within very little time. Hence we use 'little' and not 'a little'.
So, option C is correct and D is incorrect.
Option A: 'Small' expresses size and not time.
Option B: 'Few' quantifies countable nouns whereas 'time' is uncountable.
Hence options A and B are incorrect.