This is a broad question. Focus your answer on successful client relationships you've had or still have. If you've done freelancing, you'll have plenty of client relationships to potentially choose as examples. Make sure you explain why they are successful relationships by talking about how you established the relationship and how you nurture it. You should discuss communication, working through design iterations and repeat business.

Establishing a solid client relationship is critical to maintaining business and producing outstanding design work. I like to spend a lot of time getting to know a client and understanding their expectations. Once I have that established, I can really create work that will help them be successful in their goals. I make sure I stay in touch with updates on their project and maintain clear and open communication about what is and isn't working with my deliverables. Once I'm familiar with their goals and needs, I can offer up other design solutions that they may not have considered.