What project did you have the most difficult time with? What made it difficult?

"The most complicated project that I have worked on was when I was working as a junior CAD drafter for XYZ. I had little experience and the time to conduct research on particular engineering functions but in the end, was very excited about the new knowledge that I gained in the process."

Interviewers want to get more information on how well you can manage projects and challenging situations. They want to assess your approach to dealing with situations at work and how your skills could be of value in the position that you're applying for.