The correct answer is a good conductor of heat.

Important Points

• On heating one end of the iron rod, other ends also get heated. Such materials are called the good conductor of heat.
• Good conductors of heat are those materials that allow electricity or heat energy to pass through them with low resistance. Example: Gold, silver, copper, etc.
• Conduction is the mechanism of transfer of heat from the hotter end to the colder end of an object due to their temperature difference. 
• Convection is a mode of heat transfer by the actual motion of matter. It is possible only in fluids.

Additional Information

• Good Insulators of heat are those materials that do not allow heat energy to pass through them. Example: Wood, rubber, plastic, etc.
• Materials that have a conductivity between conductors and insulators are known as Semiconductors. Example: Silicon, gallium arsenide, Germanium, etc.