The rule given says that once parties reduce a transaction to writing, the terms of that writing can only be proved by producing the writing itself, and oral evidence to prove the same terms is shut out. Here, A gave B a written receipt for money paid, and someone now wants to prove that payment with oral evidence instead of the receipt.
The receipt is a written record of the payment, so its contents must be proved by the receipt itself. Oral evidence cannot substitute for it.
Therefore, the correct answer is Oral evidence to prove payment is not allowed.
The rule given establishes a hierarchy between two kinds of proof: once a transaction is captured in a document, the document becomes the primary evidence of its own terms, and oral testimony is treated as an inferior, excluded substitute for it, not an alternative path to the same conclusion. Testing each option against this idea of a hierarchy, where the writing always outranks oral testimony for its own contents, tells us which one is correct.
Because the receipt sits at the top of the hierarchy the rule creates for proving the terms of a written transaction, oral testimony cannot be used in its place to prove the payment.
Therefore, the correct answer is Oral evidence to prove payment is not allowed.