Which of the following is known as marsh gas? 

Methane 

The correct answer is option 1. Methane.

The gas known as marsh gas is Methane. Methane is commonly produced in marshes, swamps, and other wetland environments as a result of anaerobic (oxygen-free) decay of organic matter. This process is known as methanogenesis, and methane produced in such environments is often referred to as marsh gas due to its association with these habitats. Therefore, the correct answer is: Methane.