The employer would like to know what methods and resources you are using in your job search so they can determine which of their advertising methods is working best. You can expand your answer to include any other exposure you have had to the company.
"I initially saw your job posting on LinkedIn. However, I have been exposed to your organization many times before through your radio commercials and advertisements on Facebook."
"I've been looking at job postings on Indeed, so I believe this is where I found your job posting."
"Word of mouth is the most powerful form of advertising! A friend that works in your Finance Department referred me here. We worked together many years back."
"ZipRecruiter has been my new favorite place to look for fresh opportunities, and that is where I came across yours. Before seeing your job posting, I was familiar with your agency through some of your advertising efforts."
"For the last eight months, I have been contemplating a move and keeping my eye out for the perfect opening. When I saw this position open on your website, I knew I had to go for it, and I am so happy I did!"
"I heard your CEO speak at a networking event and the company sounded like a perfect fit, so I've been keeping my eye open for a job posting on your site, and now here we are."
"A colleague of mine saw your posting on a teaching forum and told me about it. He knew that my wife and I were discussing relocation to your area."
"I initially saw your job posting on LinkedIn. However, I have been exposed to your organization many times before through your radio commercials and advertisements on Facebook."
"I was really excited when I first saw your job posting because I have had an eye out for openings in your company for quite some time. Your organization has a very strong reputation and when I read the posting, it described the exact role I have been looking for. I applied immediately and am thrilled to be here interviewing with you today."