Let the client know that you are inexperienced and then do your best to advise them using the resources that you have available to you. Do not advise on anything you are really unsure about as you may be liable, an unsatisfied client would be preferable to a misinformed client. Give the client relevant contact details and reassure them that a colleague will be able to assist them fully very soon.
This question is seeking to try and find out about your personality and your judicial views as well as basic legislative knowledge. The law that you answer with may also let the interviewer know about what sort of law that you are interested in. A good choice could be a law that is in the news a lot at the moment, this will show that you are up to date with current legal issues. Try not to pick a law that is too controversial or charged as this could lead the interviewer to have a bad opinion of you.
Do not take more work than you believe you can do. Explain the situation when you are letting somebody down, and explain that you would rather not let them down by accepting work that you did not have time to complete. Once you have said you will do something it is normally a good idea to follow through and do it. This answer will show to an interviewer that you have integrity and the confidence to use your communication skills.