Answer
Sunlight warming a concrete sidewalk.
Step-by-step explanation
The transfer of thermal energy through air molecules is known as radiation.
It is as opposed to conduction (transfer through solids) and convection (transfer through liquid).
When sunlight warms a concrete sidewalk, thermal energy from the sun travels through the air to reach the sidewalk.
The option that best describes energy transfer via radiation would be sunlight warming a concrete sidewalk.