Final answer:
The object that travels at a third of the speed of light and heats the air up to 50,000 degrees is a Meteor. Bullets, rockets, and spacecrafts do not achieve such speeds nor cause extreme heating of the air in their path to those temperatures.
Step-by-step explanation:
The object that travels a third of the speed of light and can heat the air in its path up to 50,000 degrees is C) Meteor.
Meteors, when they enter Earth's atmosphere, can reach incredibly high speeds, often around 30% of the speed of light. Due to the immense friction and pressure they encounter, they heat up the surrounding air to extreme temperatures as they burn up, sometimes reaching 50,000 degrees Fahrenheit.
In comparison, a bullet moves much slower than the speed of light, and although it can get hot due to friction, it does not heat the air to such extreme temperatures. A rocket or a spacecraft might reach high speeds, but not as high as a third of the speed of light under current technological constraints. Moreover, they are designed to minimize atmospheric heating during launch and re-entry rather than causing air to reach temperatures anywhere near those produced by meteors.