Final answer:
The truck frame should typically be cut at 90 degrees to the frame centerline to ensure a strong and uniform connection when extending the frame.
Step-by-step explanation:
The question regarding the angle at which a truck frame should be cut is not exactly represented in the reference information provided since it relates to physics problems involving angles of inclines and projectile motion. However, given common engineering practices, the most structurally sound method to lengthen a truck frame is typically to cut the frame at a 90 degrees to the frame centerline, answer choice (c). This allows for a uniform and strong connection when welding an additional section of frame. The other angles, while possible, can create further complications and may not provide the same strength and structural integrity as a perpendicular cut.