Thermal radiation is the correct answer. In thermal conduction, heat is transferred through a substance via direct contact between molecules. It does not involve the movement of heatwaves along a straight line with the speed of light. Thermal convection, on the other hand, is the transfer of heat through the movement of fluid (liquid or gas) particles. It also does not involve the transmission of heatwaves in a straight line at the speed of light.
Thermal radiation is the process by which heat energy is transferred through electromagnetic waves, such as infrared radiation. Heatwaves travel in a straight line with the speed of light during thermal radiation. This means that heat can be transmitted from a source to a receiving object at the speed of light without the need for any medium. This phenomenon is commonly observed when we feel the warmth of the Sun`s rays, even from a considerable distance away.
Therefore, the correct answer is option 3: Thermal radiation.