Isn't there an "expected outcomes" document detailing what candidates are expected to know for their A and O levels? It might differ from country to country. Check your country/district/state's exam board website for that.
As for "most probable", I don't think anyone here could help you with that. Take a look through the past year examinations. Think you could spot any patterns in the questions tested? Regardless, the best person to answer this would be your high school teachers, especially those old and experienced ones. I've had a number of them tell me in advance which topics they usually expect to be tested for each year.