The correct answer is Radiation.

Concept:

• Heat is a form of energy that is measured in terms of Joule or calories.
• Heat is transferred from a hotter body to a colder body when there is a temperature difference.
• There are three modes of heat transfer:
  • Conduction, convection, and radiation.
• Heat transfer may occur with or without any medium.

Explanation:

• Conduction:
  • It is a mode of transfer of heat which occurs when the molecules collide with each other.
  • During collisions, the heat from one molecule is transferred to the others.
  • There is direct contact with the nearer molecules without the actual movement of the molecules.
  • The process of conduction takes place in solids and only because the molecules are closer in them and their movement is restricted.
• Convection:
  • It is a mode of transfer of heat where there is the actual movement of molecules.
  • The molecules move and transfer heat to the other molecules.
  • This type of heat transfer occurs in liquids and gases because the movement of the molecules is not restricted.
• Radiation:
  • The transfer of heat which doesn't require any mode for its transfer is known as Radiation.
  • Heat transfer occurs via thermal emission.
  • The heat that comes from the sun to our earth reaches via radiation.