The principle says the terms of a written contract can only be proved by producing the written contract itself, and oral evidence of those terms is excluded. A gave B a receipt after B paid him money, and oral evidence was later offered to prove that the payment had actually been made.
Since the receipt is only a record of the fact that payment was made rather than the contract itself, oral evidence remains available to prove that underlying fact, and the exclusion in the principle does not reach that far.
Therefore, the correct answer is Oral evidence to prove payment is allowed.
The fact that decides this question is what kind of document a receipt actually is, an acknowledgment that a payment already happened, not a contract laying out ongoing terms. The principle excludes oral evidence only of a written contract's terms, so each option should be tested against that distinction.
Testing each option against the distinction between a contract's terms and a mere acknowledgment of payment leaves only the option that keeps oral evidence of payment open.
Therefore, the correct answer is Oral evidence to prove payment is allowed.