Relevance is one of the hardest factors for law students to appreciate at the start of their course. You will get better at it over time, particularly if you always ask yourself why you want to tell the reader what you are telling him/her. Too often students discuss an issue because they think it might be relevant, rather than having made up their minds about what the answer really is. In some ways, relevance becomes a way to show true understanding and that is why it is so hard to achieve at the outset of your studies.
Some general guidance can be given. You should never start your answer with a general outline of the law in a particular area. You are being asked to apply the law to the particular facts you have been given, not invited to discuss everything you know about the particular area. Launch straight into the first issue arising on the facts of this case.
You should then focus on the problematic issues in the case. If you spend too much time on things that are secondary, obvious, or not even in the question you will lose valuable time that must be spent on the crucial parts that will get you marks. Although it may be difficult at first to establish what are the most problematic or important parts of the question, this is something that will get easier as you improve your knowledge of the area and gain greater experience in the law more generally.
Also, remember that the exam questions are testing your knowledge of the Cambridge Tripos Criminal Law syllabus. Therefore, if the facts raise evidence of a crime that has not been covered in the course, you do not need to scour your statute book to try to find out more about it. Mention it briefly, by all means, but do not concentrate on it. For example, in the Type B example, the question mentions the viewing of hard core pornography. Whilst there are offences relating to obscene material and computer crime, this is not part of the syllabus and you should not concern yourself with trying to find out about it. Likewise, in the Type C question, you should not spend time discussing offences relating to possession of fire arms and offensive weapons, which are not on the syllabus.