A professional's proudest accomplishment will be an indicator of what they consider a marker of their success. How you answer will inform your interviewer of what success looks like to you, which should reveal your experience level and your work ethic. It will also help them gauge your sense of accomplishment against their expectations. Whether your accomplishment is big or small, be sure to add context to why this achievement was significant in ways that showcase your professional worth. As they state on their website; "We attract, develop and retain outstanding employees by making decisions about recruitment, training, promotions and discipline on the basis of merit."