Which of the following is not commonly used as a fumigant?

Phosphorus pentoxide is indeed not commonly used as a fumigant. It primarily serves as a powerful desiccant and dehydrating agent rather than a fumigant. In pest control and food preservation, fumigants are specifically chosen for their ability to penetrate and act on pests within enclosed spaces.

Ethylene oxide, hydrogen cyanide, and methyl bromide are all established fumigants used for their efficacy in controlling pests and pathogens. Ethylene oxide is often used to sterilize medical equipment due to its ability to penetrate materials and kill microorganisms. Hydrogen cyanide is highly toxic and utilized in certain applications such as in the fumigation of structures and goods. Methyl bromide has been historically important for its use in agricultural fumigation but is being phased out due to environmental concerns.

In contrast, phosphorus pentoxide lacks these properties that make the other substances suitable for fumigation, which is why it is the correct answer in this context.