Assume each writing is signed by its maker. Which wording lacks the undertaking to pay required for a promissory note under section 4?

Section 4 of the Negotiable Instruments Act, 1881 requires an unconditional undertaking to pay, and its illustrations show that "Mr B, I.O.U. Rs 1,000" is a mere acknowledgment of debt, not a promissory note. By contrast, an acknowledgment of debt that adds "to be paid on demand, for value received" contains an undertaking to pay and is a promissory note, as are the two plain promises to pay.