According to one of the let's plays, there are two branches: green tech and civil planner. For the green tech one, since it seems to be more engineering focused as it involves creating new technologies, one guess it is physics. However, there is a small possibility it might be biology in so far as being akin to environmental studies or bioscience.
The civil planner branch appears to be most likely urban planning (since it also unlocks enthuse about city planning interaction). IRL, these jobs usually require degrees in town planning, geography or architecture but since none of those are in game, the degree will likely be most likely economics, or to lesser extent communications.
