Which of the following is a suspension system defect?

A broken leaf in a leaf spring is indeed a suspension system defect because it directly affects the vehicle's ability to support its weight and absorb shocks from the road. The leaf spring is a critical part of the suspension system, as it helps maintain proper ride height and provides stability while driving. When a leaf spring is broken, it can lead to poor handling, uneven tire wear, and an uncomfortable ride, as well as potentially putting other vehicle components at risk due to improper load distribution. This condition compromises the vehicle's safety and performance significantly, making it crucial to address immediately.

Other options, while important components of vehicle operation and safety, are not classified as suspension system defects. Worn out brake pads relate to the braking system, a loose steering wheel affects steering control, and a flat tire concerns the tire condition rather than the suspension.