Like #2 and #3, I would say the "Three G's" are the broadest reasons for exploration. I think every student must learn that mnemonic device. I know I did when I was in school, and when I taught U.S. History and World History, I used it with my students too!
If I had to rate the three reasons in terms of which one was the biggest motivating factor, I would definitely say gold. Trade, expansion, and colonialism drove exploration; particularly because the companies and countries who financed the voyages were hoping for large returns in terms of monetary compensation.