Without any context it is indeed impossible to point out a winner. There are more things to consider than just the technical, performance related questions. Like
- Location (can the tasks run on the same machine?)
- Are the tasks depending on each other in terms of execution order or data needs?
- The responsibilities for the tasks at hand (who is to do or authorize them?)
- Security issues (accessibility to input and output).
These can all impact your choice. A process is something different than a thread from an organizational point of view. A thread is an execution path within a process so they do not compare well. It is like asking which are better, desktop applications or web applications?
Let's not be too smart over this and focus on performance issues for a moment. Now we still have some questions to answer.
Those 100 execution paths, are they going to be equally busy? It is not uncommon to have a lot of threads in a single process but typically most would be asleep. It is not effective to have them all compete for CPU cycles at the same time so if you have this many threads you want to make sure that no more than some (depending on the number of cores you have to spare) will run at the same time. There are ways to do that. If tasks are basically independent from one another and you just want to do the work as quickly as possible, using threads this way is hard to beat... for a single machine scenario. Because this does not scale well.
Processes can be distributed over an unlimited number of machines, which scales a lot better when just looking at the number of execution paths you could possibly follow simultaneously. But then the question would be where the data is coming from and if there is an issue with distributing input data for those threads. It is impossible to say what the bottleneck is going to be without further context.
So it may have been a smart question with the sole purpose of seeing how strong you are when it comes to getting the information you need to do a good job. You get an unworkable request, what do you do? Will you jump to conclusions and bang away based on assumptions (or without a clue), or will you be a pain in the ass and keep asking until you understand exactly what your client needs? This would tell them something about your understanding of computer science and about whether your personality is up do the challenges typical to their environment.