I would go with 'On'.. and You could say 'The drug attached to the clothes' however it would make more sense as a sentence apart from the first or you could also use another word like 'stuck' to the clothes instead as a more relative or suitable word concerning this sentence. :)

The since the drugs are attached to the clothes the correct answer is ON.

Assuming the drugs are not INSIDE the pockets and are ON the surface of the clothes, the correct answer is ON.

🤣😂 That "THE" shouldn't be there, it was a mistake.