Explanation:

All statements are correct.

• Large but poorly known amounts of methane are trapped in the sediments beneath the sea floor,  frozen into a form of water ice called methane hydrate.

• At low temperatures the methane hydrates on the sea floor are stable, but if the water and the sea floor become warmer, then the hydrates can break down. Because microorganisms then oxidize the resulting methane gas to form the greenhouse gas carbon dioxide (CO2), methane hydrates have recently become a topic of intense discussion within the context of climate change.

• Methane is relatively short-lived in the atmosphere; a molecule of methane is oxidized to water and carbon dioxide within a decade or so, mainly by reaction with another trace gas, the hydroxyl radical OH-. Thus, unlike the case of carbon dioxide (which stays in the atmosphere longer than methane), a concerted effort to reduce methane emissions would have almost immediate results in terms of reduction of greenhouse effect. 

Hence option 4th is correct.