The correct answer is: C. When two different rail sections are required to be joined.
Staggered joints are used to join two different rail sections together. This is done to reduce the stress on the rails and to prevent them from breaking. The joints are staggered so that the load is evenly distributed across the rails.
Option A is incorrect because staggered joints are not typically used on curves. This is because the curves put a lot of stress on the rails, and the staggered joints would only make the problem worse.
Option B is incorrect because staggered joints are not typically used on straight track. This is because the straight track does not put a lot of stress on the rails, and the staggered joints would not be necessary.
Option D is incorrect because staggered joints are used to join two different rail sections together.