By which mechanism of heat transfer does heat travel in a straight line at the speed of light?

Heat travels in a straight line at the speed of light through the mechanism of radiation. This process involves the transfer of energy through electromagnetic waves, which do not require a medium to propagate. This means that heat can be transferred through the vacuum of space, allowing it to travel quickly and efficiently from one location to another, such as the heat from the sun reaching the Earth.

Radiation is distinct from conduction and convection; conduction involves direct contact between materials for heat transfer, while convection relies on the movement of fluids (liquids and gases) to distribute heat. The key characteristic of radiation is its ability to transfer energy through empty space, which is why it is the correct answer in this context.