Argemone oil is a commonly used adulterant in mustard oil. Adulteration refers to the addition of impurities or lower quality substances to a product in order to increase profits or deceive consumers. In the case of mustard oil, argemone oil is often added.

Butter, coconut oil, and diesel are not commonly adulterated with argemone oil. Butter is a dairy product, coconut oil is derived from coconuts, and diesel is a type of fuel. Argemone oil is not used to adulterate these products.

On the other hand, mustard oil, which is derived from mustard seeds, is susceptible to adulteration with argemone oil. This is because argemone oil is cheaper compared to mustard oil, and adding it can increase the quantity or volume of the product while reducing its cost. Adulterating mustard oil with argemone oil is an unethical practice as argemone oil contains toxic compounds that can harm human health.

In conclusion, argemone oil is commonly used to adulterate mustard oil, but it is not used to adulterate butter, coconut oil, or diesel.