The correct answer is: B. They contain free electrons.
Metals are good conductors of electricity because they contain free electrons. These electrons are not bound to any particular atom and can move freely throughout the metal. When a voltage is applied to a metal, these free electrons move in the direction of the voltage, creating an electric current.
The other options are not correct. Option A is incorrect because the atoms in a metal are not lightly packed. In fact, the atoms in a metal are very close together. Option C is incorrect because the melting point of a metal does not affect its ability to conduct electricity. Some metals, such as mercury, have low melting points but are still good conductors of electricity.
In conclusion, metals are good conductors of electricity because they contain free electrons. These electrons are not bound to any particular atom and can move freely throughout the metal. When a voltage is applied to a metal, these free electrons move in the direction of the voltage, creating an electric current.