Your answer could begin with a brief account of the context for this debate. It was on the radio in 1947 between two greats of their day. However, since the question is specifically on Copleston’s position in the debate, no credit is likely to be given for any of Russell’s response until you respond to Part b. In Part a, you might outline the four stages to Copleston’s argument:
The world is the sum total of all objects. None of these objects contains in itself the reason for its own existence.
If everything within the world requires something else to exist, the cause of the entire universe must be external to the universe.
This explanation must be a being that exists, but which contains within itself the cause of its existence. Its existence is ‘self-explanatory’.
You could explain the aim of any of these parts as you go along or you could write an overall explanation at the end.
This is one of those evaluation questions where some AO1 material (knowledge and understanding) will be needed in order to assess Russell’s response. So you should make a quick statement of his response in terms of denying the validity of Copleston’s terminology, stating that the universe is neither contingent nor non-contingent. Hence the chess-game comments in the radio debate. Then you could evaluate the success or otherwise of the use of the fallacy of composition. Again, your own justified reflection on this is required; there is no ‘correct’ answer.