Which of the following gases is naturally formed via the anaerobic decay of phosphorus containing organic matter?

Phosphine

The correct answer is option 3. Phosphine.

The gas that is naturally formed via the anaerobic decay of phosphorus-containing organic matter is Phosphine.

Phosphine \((PH_3)\) can be produced during the anaerobic decay of organic matter containing phosphorus. This process can occur in environments where organic material undergoes decomposition in the absence of oxygen, such as in certain sediments or sludges.