Companies will often ask this question because they want to know which of their candidate attraction strategies are delivering the best results. They also ask because they would like to understand if an employee referral is involved. Employee referral based hires are statistically more successful, as they are happier employees who stay longer. You can be brief and to the point in your reply.
"I learned about this role from Indeed.com when searching for internship opportunities. I believe the keywords that I used were 'recent graduate' and 'business development.' I hope that information is helpful to you!"
"This position was sent to me by a former colleague who knew that I was looking for a role within the oil and energy industry. I believe she saw your job posting on an industry-specific job board. I can find the details for you if you would like."
"I was referred to this position by your employee, John Doe. He mentioned to me that you were searching externally for this floor manager position and thought of me right away."
"A recruiter from ABC Recruitment firm approached me regarding this role. After meeting with her and reviewing all of the details I knew this position was a dream come true for me!"
"When shopping in the mall last weekend, I noticed the 'Now Hiring' sign in your window. I came by the next day and dropped my resume off with your assistant manager. Thank you for seeing me so quickly - that was just last week!"
"I worked with one of your clients on a major project a couple of months ago, and he mentioned that your company was hiring. This particular person knew that I was looking for career growth and a change. I am thrilled he thought of me."
"I applied to your school shortly after relocating to this city. This school is just a few blocks from my home. The location is convenient, but I have also heard great things through the teaching community here."