AS is basically the first year of the two year A level course (year 12) and the A level course is the Advanced level qualification (year 13) offered across a range of subjects to school leavers. Usually the A levels is studied over a two year period and the subjects for them have recently undergone some changes, moving from ongoing module assessments to 100% final exams (linear).

Students usually choose three or four subjects out of the 80 subjects offered, but the available subjects depend on what the main centre can provide. For Sudan it is the British Council, Khartoum and you must first check with them what subjects you can sit for.  

The subjects should also be relevant to the requirements of the university/faculty you are considering applying for.