The correct answer is: Aluminium.
Aluminium is a good conductor of electricity and is also lightweight, making it ideal for use in the armour of high voltage cables. It is also resistant to corrosion, which is important in a cable that will be exposed to the elements.
Steel is also a good conductor of electricity, but it is heavier than aluminium and is more susceptible to corrosion. Brass is not a good conductor of electricity and is not suitable for use in the armour of high voltage cables. Copper is a good conductor of electricity, but it is more expensive than aluminium and is not as lightweight.
In conclusion, aluminium is the best material for use in the armour of high voltage cables because it is a good conductor of electricity, lightweight, and resistant to corrosion.