A client needs a task done and you don't have the skills to implement it. How would you respond to the request?
If you don't have the skills it depends a bit on the context. If you're a freelancer you can take the job and subcontract the bits you can't do. I take on anything for enough money, and some projects I've basically just managed subcontractors who did the actual work. Many consulting firms do exactly this.
If it's a solo project then you realistically need to let the client know that you can't do it, otherwise you're just setting yourself up to fail.
If you aren't able to perform the task, simply do not take it on. You have the right to decline business at your discretion, so simply utilize that right.
Something along the lines of
Hello Sir/Madam, thank you for getting in touch.
I've read over your requirements and have concluded that I'd need to decline this request as it isn't 100% aligned with the services I offer.
It appears this is an interview question. So, you have an employer and the company probably has other employees too, and presumably a manager. (i.e. you're not a freelancer)
For some reason the client is asking you directly, which does happen. In this case, what I would do is tell the client that you'll discuss it with your manager (if it's a phone conversation... if it's an email you can skip this step).
Tell the manager you don't have the necessary skill. There may be another employee who can do it. It's up to the manager to either find another employee who can do it, subcontract it, give you time to learn how to do it, or tell the client that you won't be doing it / negotiate it with the client.