Which system of a vehicle must be operational to comply with Pennsylvania's noise regulations?

The muffler and exhaust system of a vehicle is critical for ensuring compliance with Pennsylvania's noise regulations. This system is designed to minimize the noise produced by the engine and reduce harmful emissions. A properly functioning muffler and exhaust system helps to maintain acceptable sound levels, preventing excessive noise that may disturb the peace or create a nuisance.

In addition to noise regulation, the exhaust system plays a vital role in controlling emissions, promoting environmental safety, and ensuring they meet state mandates. Therefore, without a well-maintained muffler and exhaust system, a vehicle could not only violate noise standards but also contribute to higher pollution levels, further affecting the surrounding environment.

The other options, while important for vehicle functionality, do not directly relate to noise compliance. The radio system and air conditioning system are primarily concerned with comfort and entertainment, while the ignition system is essential for starting the vehicle and ensuring it runs. However, it's the exhaust system that plays a pivotal role in noise regulation and compliance with state laws.