When an interviewer asks this question, remember that there are two possibilities-they may either need clarification if you single-handedly gain the accomplishment. They may point to any act in such cases. Or they want to know about your best achievement. Either way, you must walk them through your best action. Explain if you worked in a team or led one and what drove you to achieve it. For instance, "I would quote achieving 4x impressions for the previous company I worked in as the best accomplishment from my resume. I worked in a team of four other junior marketers who jointly drove arriving at such a figure".