The interviewer asks this question to understand your level of experience with selecting or creating the right marketing technology for a client's needs. From your answer, they'll try to determine if you already have skills that will allow you to immediately be an asset to them in this area of their process.

To answer this question, you should first confirm that you have experienced working with a client that required a custom solution in the past. In summarized detail, you should explain what your role in the client's project was, their technology need, and how you determined that it would require a custom solution. You should concisely explain who else was involved to build the solution - analysts, developers, or 3 party vendors, e.g.

Finally, explain how you were able to test the solution and successfully roll it out for use with the client.

Do not talk about projects where you had minimal involvement or focus on team efforts saying 'we' when referring to action taken. Say 'I' and talk about your direct involvement in a project. The interviewer wants to hear about your contribution and expertise.