What would you do if you were assigned a task which you have no experience completing?

No matter what job you have, someone is bound to give you a task that you have no experience doing. This task may be beyond your level of expertise. By asking this question, an employer is aiming to learn more about how you use your soft skills to respond to new challenges.
In your answer, you should clearly talk about how you work on developing new skills in response to new professional challenges.

Once, the head of marketing asked me to perform a complete site audit and create a report on how to improve the site. While I had done some site auditing work in the past, I was still learning the ropes.

In response to this situation, I explained to my manager that I was not ready to take on this challenge. I asked my manager if there were any professional development courses I could take that would prepare me for the job in the future. This resulted in me spending the next two weeks taking an online company course on advanced SEO. The head of marketing also mentored me as I navigated this task.

I was able to conduct the site audit to a high standard and was named the Employee of the Month for my dedication.