In a major key, major triads can be built on the first, fourth, and fifth scale degrees. Therefore, the correct answer is all of the above.
To elaborate:
- The first scale degree (I) is a major triad.
- The fourth scale degree (IV) is also a major triad.
- The fifth scale degree (V) is a major triad as well.
So, all of these scale degrees can form major triads in a major key.