Refer to figure. 
Thermal radiation is the transfer of heat from a hotter object to a cooler one by electromagnetic waves, mainly in the infrared part of the spectrum. Every object with a temperature above absolute zero gives off this energy because its particles are always moving. 

Unlike conduction and convection, thermal radiation does not need air or any other substance to travel. It can move through empty space, which is why heat from the Sun can reach the Earth. The energy travels very quickly and warms objects when it is absorbed. 

Thermal radiation is not linked to radioactivity and does not require objects to be in contact. How strong it is depends mainly on how hot the object is and the type of surface it has.

• “It is one of the many ways heat can be transferred from a distant object to another through a vacuum.” → INCORRECT. While radiation can travel through a vacuum, it is actually the only way heat can be transferred through a vacuum. Conduction and convection both require a medium (e.g. air or water).
• “The objects need to be in contact for radiation to transfer heat.” → INCORRECT. This describes conduction, not radiation.
• “It is electromagnetic radiation.” → CORRECT. As stated in the explanation above, thermal radiation is energy emitted as electromagnetic waves (mainly infrared).
• “It is radioactive radiation.” → INCORRECT. Thermal radiation is electromagnetic radiation, not radioactive radiation.