Explanation:

- Option:1, conditional

- A conditional undertaking means the promise is dependent on some event or situation. Promissory notes do not typically involve conditions for payment.

- Option:2, unconditional

- The promise in a promissory note is generally unconditional, meaning it must be paid without any preconditions.

- Option:3, maybe conditional or unconditional depending upon the circumstances

- This suggests variability based on different scenarios, but promissory notes are usually straightforward in their terms.

- Option:4, none of the above

- This would imply that none of the given options correctly describe the nature of the undertaking in a promissory note, which is incorrect.