Talk to the potential supervisors. Chat with their past and current students. Find out what projects they have in mind. Those aspects and how supportive the supervisor is are more important than the field. Pick what ever interests you most. And learn. All of those fields are interrelated and any one will be a good grounding for your future. I run a HPC for researchers and I started off in Physics. HPC's now, or will, utilise knowledge and technology from all of those fields. And importantly, have fun learning.