Choosing a domain name can be easy or it can be very difficult. It all depends on what you're trying to do and there are no wrong or right answers. In this task you're going to pick a domain name that feels right for your portfolio. You can have any domain name that is not already registered. You can use sites like domai.nr to check the availabilty of a domain name.
Many people choose to register their own name as the domain name for their portfolio. This approach may not be the most exciting method, but it is very versatile and to the point.
You may choose to register a word with specific meaning behind it. Jeremy Keith uses adactio.com for his domain and this may be for its latin meaning or it may simply be because he likes it.
Domain hacking is another popular method for choosing a domain name. In this sense the term hacking refers to the art of combining all the parts of the domain to form an overall word.
Bit.ly, del.icio.us and foundat.io/n are examples of domain hacking. Typically it costs a little bit more than a .com to register a domain suitable for hacking (there are often more legalities to it as well) so you might want to bear this in mind when selecting a name.
Spend the next 20 minutes or so thinking of a domain name and checking its availability. If you find one that you think is perfect for your needs then go ahead and register it. If you can't think of one in this time then don't register one just to finish the task. You can finish this task in your own time. The important part is that you register a domain name that you're happy with.
When you've registered your domain name, post it onto the challenge discussion page telling people what your domain name is.