What can be a symptom of air brake failure?

Answer: C — Loss of brake pressure

Loss of brake pressure is a critical symptom of air brake failure because it directly affects the braking system's ability to function correctly. Air brakes rely on a consistent supply of compressed air to engage the braking mechanism effectively. When there is a loss of brake pressure, it may indicate a leak in the system or failure of the components that provide and maintain that pressure. This can lead to insufficient braking force, making it difficult for the train to stop safely, which poses a serious safety risk. In contrast to this, normal brake operation would indicate that the brakes are functioning as they are supposed to; unexpected brake engagement would denote a malfunction that could also signify a problem, but is not as definitive as loss of pressure; improved stopping distance contradicts the principles of brake operation altogether, as a loss of pressure typically leads to longer stopping distances.