My guess would have been computing, as studied by Bill Gates and Mark Zuckerberg, but it isn’t. Both these bright sparks dropped out to launch their startups.
It’s not physics, accounting, or engineering either.
The answer is economics.
Elon Musk graduated with a Bachelor of Science in Physics, but he also has a Bachelor of Arts in Economics. Steve Cohen, Meg Whitman, and Bob Kraft all studied economics in college.