a(i) Copper (or any metal).
Copper, being a metal, is a good conductor of heat. The heat generated by the electronic component will be conducted away at a faster rate by the heat sink, preventing the component from overheating.
ii) Black color.
Black colored surfaces are better emitter of radiant heat than bright colored surfaces. This will help the heat sink to emit its heat to the surroundings at a faster rate, so that the heat sink do not get too hot, and will still be able to gain heat from the electronic component effectively.

b)(i) 1W –> increase by 2 deg C
so 10W –> increase by 20 deg C
Answer: 20 deg C
b)(ii) As the temperature of the heat sink increase, the rate of heat conduction from electronic component will slow down, and the rate of heat radiated to surroundings will increase. Eventually, the rate of heat conducted into the heat sink will be equal to the rate of heat emitted from it. At this point, its temperature will remain constant.