Chaos wouldn't suit humans (if anyone thinks it does, they should justify it), and darkness doesn't really fit in a city where they worship a dragon of light (if I'm not mistaken, this was addressed in later installments of Heroes, where darkness is not available to them at all, especially in Heroes 7, unless something has changed). So, light and summoning (and even summoning includes various celestial shields) – summoning clearly suits them more than darkness (Elrath wouldn't approve of studying darkness:D, they are all fanatical followers of the church of light).

Added 5 minutes later
Oh yes, and dwarves: chaos-summoning is already used by mages, and chaos-light isn't used by anyone, which is why it suits dwarves, besides, it's also chaos-light in the original game.

Added 8 minutes later


From the proposed options:
darkness-chaos:-----------Inferno---------Dungeon-----
light-summoning:----------Humans---------Sylvan--
chaos-summoning:--------Academy---------------------
darkness-summoning---------Necropolis--------------------
chaos-light:--------Fortress----------------

we see:
- summoning 4 times
- chaos 4 times
- darkness 3 times
- light 3 times
so summoning is not more frequent than chaos