Which of the following gases is NOT commonly used in fumigation?

In the context of fumigation, the substances commonly employed are specifically chosen for their effectiveness in controlling pests while being appropriate for application under controlled circumstances. Methyl bromide, phosphine, and sulfuryl fluoride are all well-established fumigants due to their efficacy in penetrating materials and affecting pests.

Methyl bromide has a long history of use, particularly for its rapid action against a wide range of pests. Phosphine, generated from metal phosphides, is commonly used because of its effectiveness against stored product pests and is favored for its low environmental impact relative to other fumigants. Sulfuryl fluoride is utilized as a structural fumigant and is effective on drywood termites and other wood-boring pests.

In contrast, carbon monoxide is not commonly used in fumigation practices because it is not an effective pest control agent in the same way as the other listed chemicals. Although it can be toxic and has industrial applications, its use as a fumigant is not standard within pest management protocols. Hence, the selection of carbon monoxide as the answer highlights an understanding of the fumigation process and the purpose of using specific gases designed for controlling pest populations effectively.