When they ask “how did you hear about the position?”, the interviewer just wants to know if you’ve taken the time to research the company and if you have a genuine reason for wanting to talk with them. Mention a product, a mission statement on the website, a reputation for talented employees, or whatever else seems applicable to that specific company. Come up with a great reason. Don’t make it seem like they’re just one company among many. Or that you’re sending your resume out to them for no particular reason other than wanting a job.
What your interviewer is really trying to do with this question—beyond identifying any major red flags—is to gauge your self-awareness and honesty. So, “I can't meet a deadline to save my life” is not an option—but neither is “Nothing! I'm perfect!” Strike a balance by thinking of something that you struggle with but that you’re working to improve. For example, maybe you’ve never been strong at public speaking, but you've recently volunteered to run meetings to help you be more comfortable when addressing a crowd.
Any candidate can read and regurgitate the company’s “About” page. So, when interviewers ask this, they aren't necessarily trying to gauge whether you understand the mission—they want to know whether you care about it. Start with one line that shows you understand the company's goals, using a couple key words and phrases from the website, but then go on to make it personal. Say, “I’m personally drawn to this mission because…” or “I really believe in this approach because…” and share a personal example or two.