Answer: Convection, because air currents are formed.
There are three modes of Heat transfer: Conduction, convection and Radiation. Within solids or when two solids are in contact, heat is transferred via conduction. In fluids (liquids and gases), heat is transferred via convection. This happens because, the fluid in contact with the heating filament is more hot than top layers. the hot layer rises and denser cool layer settles down and goes through heating. This process continues, until entire fluid is evenly heated. Hence, furnace is able to heat up the entire room through convection mode of heat transfer. Radiation is the mode of heat transfer in the form of electromagnetic waves like UV rays, gamma rays, infrared rays etc.
