A public good is a type of good that is non-excludable and non-rivalrous, meaning that it is available for everyone to use and one person`s use does not diminish its availability for others. It is typically provided by the government or a collective entity.

Option 1: Electricity - This is not a public good because it is excludable, meaning that access to electricity can be restricted and controlled by individuals or organizations. People who do not pay for electricity may not have access to it.

Option 2: National Defence - This is a public good as it is provided by the government and is available to all citizens for their protection. It is non-excludable and non-rivalrous.

Option 3: Light House - This is a public good as it is a navigational aid provided for the benefit of all ships and sailors. It is non-excludable and non-rivalrous.

Option 4: Public Parks - This is a public good as it is open to everyone and one person`s use of the park does not take away from the availability for others. It is non-excludable and non-rivalrous.

Therefore, the correct answer is option 1, electricity, as it is excludable and therefore not a public good