Talk about the differences between PR and advertising.
It's important to the interviewer that you understand and can explain how public relations and advertising differ. Prove that you're knowledgeable in this area by explaining how the two functions are different from each other. Then reiterate why you chose to work in PR instead of advertising and connect it to the role.
"Advertising and PR are similar in that they both work to build the brand of a company or organization via communication with their target audience. The big difference between the two, however, is that a company pays for advertising space whereas with PR they have to earn media mentions and recognition through pitches and press releases. In other words, advertising is paid for, while publicity is earned or, in some cases, hoped for. The reason I chose PR over advertising is that I enjoy the challenge of persuading and influencing others, whether they be in the media or a third party, to speak favorably about my organization. I also find PR to be far more effective than advertising because consumers tend to trust it more."