Unless you're applying for a job at Google or similar who are well known for being a great place to work, how on earth would you know if it's the only place for you? Until the interview you have no idea, an interview is your chance to get to know the employers as much as it is for them to get to know you. So why would an employer not expect you to be looking for other jobs? They're probably more interested in if you're serious about leaving your current position, if they take their time finding the right candidate if you'll still be available, have you had 100 unsuccessful interviews but this one just happened to click, etc. An answer such as "I have an offer from another company but this role seems more interesting to me because of X, Y & Z, I have to let them know within a week so a quick response to this interview would be much appreciated" would be perfectly acceptable if it's true. Probably best not to mention the companies involved though.