DevOps engineers are most likely to get asked this question during their technical assessment in companies like Accenture and Epam.
There are a lot of resources that explain in details what the main diferencies of these open-source web servers. You might have already read several articles and think that you are well prepared, but the stronges answer for this question is not so obvious for most of the junior DevOps engineers, even after hitting numerous posts and technical articles.
So, the strongest answer (at Epam for sure) would be that Apache synchronously processes requests, and in contrast Nginx does it asynchronously.
I found the following correct answer by the link https://softwareengineering.stackexchange.com/questions/256510/how-is-nginx-handling-its-requests-in-terms-of-tasks-or-threading
"Nginx uses an asynchronous event-driven approach to handling requests, instead of the Apache HTTP Server model that defaults to a threaded or process-oriented approach, where the Event MPM is required for asynchronous processing. Nginx's modular event-driven architecture can provide more predictable performance under high loads".
Apache - synchronous
Nginx - asynchronous
I found the article below very informative, though it doesn't contain clearely stated strong answer that your assessment committe is expecting to hear: