What is the most likely cause of high HC and CO emissions indicated by a P0455 trouble code?

The identification of a P0455 trouble code typically indicates a large leak in the evaporative emission control system, which is responsible for capturing and recycling fuel vapors from the fuel tank. A loose or missing gas cap is a common culprit for this error, as it can allow fuel vapors to escape into the atmosphere. This escape not only leads to the emission of hydrocarbons (HC) and carbon monoxide (CO) due to unburned fuel but also triggers the vehicle's onboard diagnostic system to register the P0455 code.

A loose or missing gas cap directly impacts the engine's ability to maintain the correct pressure in the fuel system, which can result in increased emissions. This scenario aligns with the symptoms of high HC and CO levels, as the unregulated fuel vapors contribute to these pollutants escaping into the exhaust system.

The other choices, such as a failing oxygen sensor, a clogged air filter, and a malfunctioning throttle position sensor, could cause various performance and emission issues but are not directly related to the indication of a large leak in the evaporative system represented by the P0455 code. Thus, recognizing the relationship between the gas cap condition and the emissions problem substantiates why it is the most likely cause.