The correct answer is (a) Mercury.
Mercury is a very good conductor of heat because it is a metal. Metals are good conductors of heat because they have free electrons that can move easily. These free electrons can carry heat energy from one part of the metal to another.
Water is also a good conductor of heat, but it is not as good as mercury. This is because water molecules are more tightly bonded than mercury atoms. The bonds between water molecules make it more difficult for heat energy to move from one water molecule to another.
Ether and benzene are both non-metals. Non-metals are not good conductors of heat because they do not have free electrons. The electrons in non-metals are tightly bound to the atoms, and they cannot move easily. This makes it difficult for heat energy to move from one part of a non-metal to another.
In conclusion, mercury is the best conductor of heat among the given options.