The correct answer is option 2: Methane. Greenhouse gases are gases that trap heat in the Earth`s atmosphere, contributing to the greenhouse effect and thus causing global warming. Methane is one of the most potent greenhouse gases, with a much higher heat-trapping potential compared to carbon dioxide. It is produced through both natural processes and human activities.

In February 2016, the largest greenhouse gas leakage in the US history occurred. While the specific event or incident that led to this leakage is not mentioned, we can infer that the gas released was methane based on the provided options. Methane leaks can occur from various sources such as natural gas production and distribution, coal mining, landfills, and agricultural activities.

It is worth noting that while carbon dioxide is the most prevalent greenhouse gas emitted by human activities, methane is significantly more potent in terms of its heat-trapping ability.

Therefore, option 2, Methane, is the correct answer.