The 1997 Indonesian forest fire was a result of deliberate human activity – slash-and-burn farming methods – rather than a natural phenomenon. Therefore, the correct answer is indeed `None of the above`. To explain other options:

Option 1: Greenhouse effect refers to thermal heating of the earth`s surface caused by certain gases in the atmosphere. While it contributes to global warming, it doesn`t directly cause forest fires.

Option 2: Depletion of the ozone layer allows more harmful UV radiation to reach the earth`s surface. This can cause skin cancer and cataract in humans and affect marine life, but it doesn`t directly cause forest fires either.

Option 3: El Nino is a climate pattern associated with warming of certain parts of the Pacific ocean. It can exacerbate forest fires, especially if they coincide with periods of drought, but it isn`t the primary cause of those fires.

Thus, the explanation verifies the correctness of the provided answer - none of the options directly caused the 1997 Indonesian forest fire.