(a) is the most correct English phrasing, probably. This would mean that the mother is very unlikely to recover, which most people would view as a negative outcome.
(b) is a /possible/ answer, but would be more commonly phrased as "a small chance of recovering". This is again a negative outcome, but in context may be slightly more positive, as in "Her mother is very hurt, but has a small chance of recovering." It is quite strange that "a little" would very rarely used in this case by a native English speaker, but, on the face of it, means exactly the same as "a small".
(c) definitely is not standard English.