One of those wonderful JAA questions where the reality is probably very different from the text book answer.
The sensible options are a or c, though c I think is the answer the examiner is probably looking for. M.85 is right though - running away is not an option and the cleaners and lawyers might not thank you for d.
With a (text book) jetstream, the worst CAT is just below the jetstream axis, on the cold side of the jetstream (on the RHS if you are flying west over the Atlantic). If you descend, you should be able to clear this turbulence. However, if you climb, you may well encounter an area of secondary CAT which is found just above the jetstream axis. A good diagram would probably make this clearer.
The problem in reality all depends on exactly where you are in relation to the jetsream in the first place (ie, in the primary or secondary CAT zones - assume primary for the question), and what options a combination of ATC and performance restrictions place upon the flight (you may not be able to climb due to weight for instance). Have fun.
