The flow of heat from one body to another, or from one place to another is called transmission of heat. There are three methods of heat transmission.
(a) Radiation: Heat is transmitted in the form of electromagnetic waves from a hot body. It does not require any medium. Hence, in this case heat is transmitted by the process of radiation.
(b) Convection: The process of heat transmission in which the particles of the fluid move is called convection. Convection takes place only in liquids and gases. It requires a medium, hence it is not the correct option.
(c) Conduction: When heat flows from the part at higher temperature to the part at lower temperature, it requires a medium for transmission, then it is known as conduction of heat. Hence, this option is also not correct. Therefore, option (a) is correct.