The correct answer is A. on.
A revenue stamp is a type of stamp that is used to pay a tax. It is usually affixed to a document, such as a contract or a license. The signature of the person who is signing the document is usually placed on top of the revenue stamp.
The other options are incorrect because they do not accurately describe the location of the signature. Option B, upo, is not a word in the English language. Option C, above, suggests that the signature is placed above the revenue stamp, which is not the case. Option D, over, suggests that the signature is placed completely covering the revenue stamp, which is also not the case.
In conclusion, the correct answer to the question “Affix a revenue stamp and put your signature …… it.” is A. on.