Petroleum forms over millions of years when dead marine organisms get buried under layers of sediment, where they are subjected to high temperature, high pressure, and an oxygen-free (absence of air) environment, which slowly converts them into petroleum. The 'presence of water' on its own is not listed as one of the core necessary conditions for this conversion process — it's the heat, pressure, and lack of air that matter most. So the correct answer is presence of water, option (3).